git-rebase: error out when incompatible options passed

[thirdparty/git.git] / Documentation / git-rebase.txt

git-rebase(1)
=============

NAME
----
git-rebase - Reapply commits on top of another base tip

SYNOPSIS
--------
[verse]
'git rebase' [-i | --interactive] [<options>] [--exec <cmd>] [--onto <newbase>]
	[<upstream> [<branch>]]
'git rebase' [-i | --interactive] [<options>] [--exec <cmd>] [--onto <newbase>]
	--root [<branch>]
'git rebase' --continue | --skip | --abort | --quit | --edit-todo | --show-current-patch

DESCRIPTION
-----------
If <branch> is specified, 'git rebase' will perform an automatic
`git checkout <branch>` before doing anything else.  Otherwise
it remains on the current branch.

If <upstream> is not specified, the upstream configured in
branch.<name>.remote and branch.<name>.merge options will be used (see
linkgit:git-config[1] for details) and the `--fork-point` option is
assumed.  If you are currently not on any branch or if the current
branch does not have a configured upstream, the rebase will abort.

All changes made by commits in the current branch but that are not
in <upstream> are saved to a temporary area.  This is the same set
of commits that would be shown by `git log <upstream>..HEAD`; or by
`git log 'fork_point'..HEAD`, if `--fork-point` is active (see the
description on `--fork-point` below); or by `git log HEAD`, if the
`--root` option is specified.

The current branch is reset to <upstream>, or <newbase> if the
--onto option was supplied.  This has the exact same effect as
`git reset --hard <upstream>` (or <newbase>).  ORIG_HEAD is set
to point at the tip of the branch before the reset.

The commits that were previously saved into the temporary area are
then reapplied to the current branch, one by one, in order. Note that
any commits in HEAD which introduce the same textual changes as a commit
in HEAD..<upstream> are omitted (i.e., a patch already accepted upstream
with a different commit message or timestamp will be skipped).

It is possible that a merge failure will prevent this process from being
completely automatic.  You will have to resolve any such merge failure
and run `git rebase --continue`.  Another option is to bypass the commit
that caused the merge failure with `git rebase --skip`.  To check out the
original <branch> and remove the .git/rebase-apply working files, use the
command `git rebase --abort` instead.

Assume the following history exists and the current branch is "topic":

------------
          A---B---C topic
         /
    D---E---F---G master
------------

From this point, the result of either of the following commands:


    git rebase master
    git rebase master topic

would be:

------------
                  A'--B'--C' topic
                 /
    D---E---F---G master
------------

*NOTE:* The latter form is just a short-hand of `git checkout topic`
followed by `git rebase master`. When rebase exits `topic` will
remain the checked-out branch.

If the upstream branch already contains a change you have made (e.g.,
because you mailed a patch which was applied upstream), then that commit
will be skipped. For example, running `git rebase master` on the
following history (in which `A'` and `A` introduce the same set of changes,
but have different committer information):

------------
          A---B---C topic
         /
    D---E---A'---F master
------------

will result in:

------------
                   B'---C' topic
                  /
    D---E---A'---F master
------------

Here is how you would transplant a topic branch based on one
branch to another, to pretend that you forked the topic branch
from the latter branch, using `rebase --onto`.

First let's assume your 'topic' is based on branch 'next'.
For example, a feature developed in 'topic' depends on some
functionality which is found in 'next'.

------------
    o---o---o---o---o  master
         \
          o---o---o---o---o  next
                           \
                            o---o---o  topic
------------

We want to make 'topic' forked from branch 'master'; for example,
because the functionality on which 'topic' depends was merged into the
more stable 'master' branch. We want our tree to look like this:

------------
    o---o---o---o---o  master
        |            \
        |             o'--o'--o'  topic
         \
          o---o---o---o---o  next
------------

We can get this using the following command:

    git rebase --onto master next topic


Another example of --onto option is to rebase part of a
branch.  If we have the following situation:

------------
                            H---I---J topicB
                           /
                  E---F---G  topicA
                 /
    A---B---C---D  master
------------

then the command

    git rebase --onto master topicA topicB

would result in:

------------
                 H'--I'--J'  topicB
                /
                | E---F---G  topicA
                |/
    A---B---C---D  master
------------

This is useful when topicB does not depend on topicA.

A range of commits could also be removed with rebase.  If we have
the following situation:

------------
    E---F---G---H---I---J  topicA
------------

then the command

    git rebase --onto topicA~5 topicA~3 topicA

would result in the removal of commits F and G:

------------
    E---H'---I'---J'  topicA
------------

This is useful if F and G were flawed in some way, or should not be
part of topicA.  Note that the argument to --onto and the <upstream>
parameter can be any valid commit-ish.

In case of conflict, 'git rebase' will stop at the first problematic commit
and leave conflict markers in the tree.  You can use 'git diff' to locate
the markers (<<<<<<) and make edits to resolve the conflict.  For each
file you edit, you need to tell Git that the conflict has been resolved,
typically this would be done with


    git add <filename>


After resolving the conflict manually and updating the index with the
desired resolution, you can continue the rebasing process with


    git rebase --continue


Alternatively, you can undo the 'git rebase' with


    git rebase --abort

CONFIGURATION
-------------

include::rebase-config.txt[]

OPTIONS
-------
--onto <newbase>::
	Starting point at which to create the new commits. If the
	--onto option is not specified, the starting point is
	<upstream>.  May be any valid commit, and not just an
	existing branch name.
+
As a special case, you may use "A\...B" as a shortcut for the
merge base of A and B if there is exactly one merge base. You can
leave out at most one of A and B, in which case it defaults to HEAD.

<upstream>::
	Upstream branch to compare against.  May be any valid commit,
	not just an existing branch name. Defaults to the configured
	upstream for the current branch.

<branch>::
	Working branch; defaults to HEAD.

--continue::
	Restart the rebasing process after having resolved a merge conflict.

--abort::
	Abort the rebase operation and reset HEAD to the original
	branch. If <branch> was provided when the rebase operation was
	started, then HEAD will be reset to <branch>. Otherwise HEAD
	will be reset to where it was when the rebase operation was
	started.

--quit::
	Abort the rebase operation but HEAD is not reset back to the
	original branch. The index and working tree are also left
	unchanged as a result.

--keep-empty::
	Keep the commits that do not change anything from its
	parents in the result.
+
See also INCOMPATIBLE OPTIONS below.

--allow-empty-message::
	By default, rebasing commits with an empty message will fail.
	This option overrides that behavior, allowing commits with empty
	messages to be rebased.
+
See also INCOMPATIBLE OPTIONS below.

--skip::
	Restart the rebasing process by skipping the current patch.

--edit-todo::
	Edit the todo list during an interactive rebase.

--show-current-patch::
	Show the current patch in an interactive rebase or when rebase
	is stopped because of conflicts. This is the equivalent of
	`git show REBASE_HEAD`.

-m::
--merge::
	Use merging strategies to rebase.  When the recursive (default) merge
	strategy is used, this allows rebase to be aware of renames on the
	upstream side.
+
Note that a rebase merge works by replaying each commit from the working
branch on top of the <upstream> branch.  Because of this, when a merge
conflict happens, the side reported as 'ours' is the so-far rebased
series, starting with <upstream>, and 'theirs' is the working branch.  In
other words, the sides are swapped.
+
See also INCOMPATIBLE OPTIONS below.

-s <strategy>::
--strategy=<strategy>::
	Use the given merge strategy.
	If there is no `-s` option 'git merge-recursive' is used
	instead.  This implies --merge.
+
Because 'git rebase' replays each commit from the working branch
on top of the <upstream> branch using the given strategy, using
the 'ours' strategy simply empties all patches from the <branch>,
which makes little sense.
+
See also INCOMPATIBLE OPTIONS below.

-X <strategy-option>::
--strategy-option=<strategy-option>::
	Pass the <strategy-option> through to the merge strategy.
	This implies `--merge` and, if no strategy has been
	specified, `-s recursive`.  Note the reversal of 'ours' and
	'theirs' as noted above for the `-m` option.
+
See also INCOMPATIBLE OPTIONS below.

-S[<keyid>]::
--gpg-sign[=<keyid>]::
	GPG-sign commits. The `keyid` argument is optional and
	defaults to the committer identity; if specified, it must be
	stuck to the option without a space.

-q::
--quiet::
	Be quiet. Implies --no-stat.

-v::
--verbose::
	Be verbose. Implies --stat.

--stat::
	Show a diffstat of what changed upstream since the last rebase. The
	diffstat is also controlled by the configuration option rebase.stat.

-n::
--no-stat::
	Do not show a diffstat as part of the rebase process.

--no-verify::
	This option bypasses the pre-rebase hook.  See also linkgit:githooks[5].

--verify::
	Allows the pre-rebase hook to run, which is the default.  This option can
	be used to override --no-verify.  See also linkgit:githooks[5].

-C<n>::
	Ensure at least <n> lines of surrounding context match before
	and after each change.  When fewer lines of surrounding
	context exist they all must match.  By default no context is
	ever ignored.
+
See also INCOMPATIBLE OPTIONS below.

-f::
--force-rebase::
	Force a rebase even if the current branch is up to date and
	the command without `--force` would return without doing anything.
+
You may find this (or --no-ff with an interactive rebase) helpful after
reverting a topic branch merge, as this option recreates the topic branch with
fresh commits so it can be remerged successfully without needing to "revert
the reversion" (see the
link:howto/revert-a-faulty-merge.html[revert-a-faulty-merge How-To] for details).

--fork-point::
--no-fork-point::
	Use reflog to find a better common ancestor between <upstream>
	and <branch> when calculating which commits have been
	introduced by <branch>.
+
When --fork-point is active, 'fork_point' will be used instead of
<upstream> to calculate the set of commits to rebase, where
'fork_point' is the result of `git merge-base --fork-point <upstream>
<branch>` command (see linkgit:git-merge-base[1]).  If 'fork_point'
ends up being empty, the <upstream> will be used as a fallback.
+
If either <upstream> or --root is given on the command line, then the
default is `--no-fork-point`, otherwise the default is `--fork-point`.

--ignore-whitespace::
--whitespace=<option>::
	These flag are passed to the 'git apply' program
	(see linkgit:git-apply[1]) that applies the patch.
+
See also INCOMPATIBLE OPTIONS below.

--committer-date-is-author-date::
--ignore-date::
	These flags are passed to 'git am' to easily change the dates
	of the rebased commits (see linkgit:git-am[1]).
+
See also INCOMPATIBLE OPTIONS below.

--signoff::
	Add a Signed-off-by: trailer to all the rebased commits. Note
	that if `--interactive` is given then only commits marked to be
	picked, edited or reworded will have the trailer added.
+
See also INCOMPATIBLE OPTIONS below.

-i::
--interactive::
	Make a list of the commits which are about to be rebased.  Let the
	user edit that list before rebasing.  This mode can also be used to
	split commits (see SPLITTING COMMITS below).
+
The commit list format can be changed by setting the configuration option
rebase.instructionFormat.  A customized instruction format will automatically
have the long commit hash prepended to the format.
+
See also INCOMPATIBLE OPTIONS below.

-r::
--rebase-merges[=(rebase-cousins|no-rebase-cousins)]::
	By default, a rebase will simply drop merge commits from the todo
	list, and put the rebased commits into a single, linear branch.
	With `--rebase-merges`, the rebase will instead try to preserve
	the branching structure within the commits that are to be rebased,
	by recreating the merge commits. Any resolved merge conflicts or
	manual amendments in these merge commits will have to be
	resolved/re-applied manually.
+
By default, or when `no-rebase-cousins` was specified, commits which do not
have `<upstream>` as direct ancestor will keep their original branch point,
i.e. commits that would be excluded by gitlink:git-log[1]'s
`--ancestry-path` option will keep their original ancestry by default. If
the `rebase-cousins` mode is turned on, such commits are instead rebased
onto `<upstream>` (or `<onto>`, if specified).
+
The `--rebase-merges` mode is similar in spirit to `--preserve-merges`, but
in contrast to that option works well in interactive rebases: commits can be
reordered, inserted and dropped at will.
+
It is currently only possible to recreate the merge commits using the
`recursive` merge strategy; Different merge strategies can be used only via
explicit `exec git merge -s <strategy> [...]` commands.
+
See also REBASING MERGES and INCOMPATIBLE OPTIONS below.

-p::
--preserve-merges::
	Recreate merge commits instead of flattening the history by replaying
	commits a merge commit introduces. Merge conflict resolutions or manual
	amendments to merge commits are not preserved.
+
This uses the `--interactive` machinery internally, but combining it
with the `--interactive` option explicitly is generally not a good
idea unless you know what you are doing (see BUGS below).
+
See also INCOMPATIBLE OPTIONS below.

-x <cmd>::
--exec <cmd>::
	Append "exec <cmd>" after each line creating a commit in the
	final history. <cmd> will be interpreted as one or more shell
	commands.
+
You may execute several commands by either using one instance of `--exec`
with several commands:
+
	git rebase -i --exec "cmd1 && cmd2 && ..."
+
or by giving more than one `--exec`:
+
	git rebase -i --exec "cmd1" --exec "cmd2" --exec ...
+
If `--autosquash` is used, "exec" lines will not be appended for
the intermediate commits, and will only appear at the end of each
squash/fixup series.
+
This uses the `--interactive` machinery internally, but it can be run
without an explicit `--interactive`.
+
See also INCOMPATIBLE OPTIONS below.

--root::
	Rebase all commits reachable from <branch>, instead of
	limiting them with an <upstream>.  This allows you to rebase
	the root commit(s) on a branch.  When used with --onto, it
	will skip changes already contained in <newbase> (instead of
	<upstream>) whereas without --onto it will operate on every change.
	When used together with both --onto and --preserve-merges,
	'all' root commits will be rewritten to have <newbase> as parent
	instead.
+
See also INCOMPATIBLE OPTIONS below.

--autosquash::
--no-autosquash::
	When the commit log message begins with "squash! ..." (or
	"fixup! ..."), and there is already a commit in the todo list that
	matches the same `...`, automatically modify the todo list of rebase
	-i so that the commit marked for squashing comes right after the
	commit to be modified, and change the action of the moved commit
	from `pick` to `squash` (or `fixup`).  A commit matches the `...` if
	the commit subject matches, or if the `...` refers to the commit's
	hash. As a fall-back, partial matches of the commit subject work,
	too.  The recommended way to create fixup/squash commits is by using
	the `--fixup`/`--squash` options of linkgit:git-commit[1].
+
If the `--autosquash` option is enabled by default using the
configuration variable `rebase.autoSquash`, this option can be
used to override and disable this setting.
+
See also INCOMPATIBLE OPTIONS below.

--autostash::
--no-autostash::
	Automatically create a temporary stash entry before the operation
	begins, and apply it after the operation ends.  This means
	that you can run rebase on a dirty worktree.  However, use
	with care: the final stash application after a successful
	rebase might result in non-trivial conflicts.

--no-ff::
	With --interactive, cherry-pick all rebased commits instead of
	fast-forwarding over the unchanged ones.  This ensures that the
	entire history of the rebased branch is composed of new commits.
+
Without --interactive, this is a synonym for --force-rebase.
+
You may find this helpful after reverting a topic branch merge, as this option
recreates the topic branch with fresh commits so it can be remerged
successfully without needing to "revert the reversion" (see the
link:howto/revert-a-faulty-merge.html[revert-a-faulty-merge How-To] for details).

INCOMPATIBLE OPTIONS
--------------------

git-rebase has many flags that are incompatible with each other,
predominantly due to the fact that it has three different underlying
implementations:

 * one based on linkgit:git-am[1] (the default)
 * one based on git-merge-recursive (merge backend)
 * one based on linkgit:git-cherry-pick[1] (interactive backend)

Flags only understood by the am backend:

 * --committer-date-is-author-date
 * --ignore-date
 * --whitespace
 * --ignore-whitespace
 * -C

Flags understood by both merge and interactive backends:

 * --merge
 * --strategy
 * --strategy-option
 * --allow-empty-message

Flags only understood by the interactive backend:

 * --[no-]autosquash
 * --rebase-merges
 * --preserve-merges
 * --interactive
 * --exec
 * --keep-empty
 * --autosquash
 * --edit-todo
 * --root when used in combination with --onto

Other incompatible flag pairs:

 * --preserve-merges and --interactive
 * --preserve-merges and --signoff
 * --preserve-merges and --rebase-merges
 * --rebase-merges and --strategy
 * --rebase-merges and --strategy-option

include::merge-strategies.txt[]

NOTES
-----

You should understand the implications of using 'git rebase' on a
repository that you share.  See also RECOVERING FROM UPSTREAM REBASE
below.

When the git-rebase command is run, it will first execute a "pre-rebase"
hook if one exists.  You can use this hook to do sanity checks and
reject the rebase if it isn't appropriate.  Please see the template
pre-rebase hook script for an example.

Upon completion, <branch> will be the current branch.

INTERACTIVE MODE
----------------

Rebasing interactively means that you have a chance to edit the commits
which are rebased.  You can reorder the commits, and you can
remove them (weeding out bad or otherwise unwanted patches).

The interactive mode is meant for this type of workflow:

1. have a wonderful idea
2. hack on the code
3. prepare a series for submission
4. submit

where point 2. consists of several instances of

a) regular use

 1. finish something worthy of a commit
 2. commit

b) independent fixup

 1. realize that something does not work
 2. fix that
 3. commit it

Sometimes the thing fixed in b.2. cannot be amended to the not-quite
perfect commit it fixes, because that commit is buried deeply in a
patch series.  That is exactly what interactive rebase is for: use it
after plenty of "a"s and "b"s, by rearranging and editing
commits, and squashing multiple commits into one.

Start it with the last commit you want to retain as-is:

	git rebase -i <after-this-commit>

An editor will be fired up with all the commits in your current branch
(ignoring merge commits), which come after the given commit.  You can
reorder the commits in this list to your heart's content, and you can
remove them.  The list looks more or less like this:

-------------------------------------------
pick deadbee The oneline of this commit
pick fa1afe1 The oneline of the next commit
...
-------------------------------------------

The oneline descriptions are purely for your pleasure; 'git rebase' will
not look at them but at the commit names ("deadbee" and "fa1afe1" in this
example), so do not delete or edit the names.

By replacing the command "pick" with the command "edit", you can tell
'git rebase' to stop after applying that commit, so that you can edit
the files and/or the commit message, amend the commit, and continue
rebasing.

If you just want to edit the commit message for a commit, replace the
command "pick" with the command "reword".

To drop a commit, replace the command "pick" with "drop", or just
delete the matching line.

If you want to fold two or more commits into one, replace the command
"pick" for the second and subsequent commits with "squash" or "fixup".
If the commits had different authors, the folded commit will be
attributed to the author of the first commit.  The suggested commit
message for the folded commit is the concatenation of the commit
messages of the first commit and of those with the "squash" command,
but omits the commit messages of commits with the "fixup" command.

'git rebase' will stop when "pick" has been replaced with "edit" or
when a command fails due to merge errors. When you are done editing
and/or resolving conflicts you can continue with `git rebase --continue`.

For example, if you want to reorder the last 5 commits, such that what
was HEAD~4 becomes the new HEAD. To achieve that, you would call
'git rebase' like this:

----------------------
$ git rebase -i HEAD~5
----------------------

And move the first patch to the end of the list.

You might want to preserve merges, if you have a history like this:

------------------
           X
            \
         A---M---B
        /
---o---O---P---Q
------------------

Suppose you want to rebase the side branch starting at "A" to "Q". Make
sure that the current HEAD is "B", and call

-----------------------------
$ git rebase -i -p --onto Q O
-----------------------------

Reordering and editing commits usually creates untested intermediate
steps.  You may want to check that your history editing did not break
anything by running a test, or at least recompiling at intermediate
points in history by using the "exec" command (shortcut "x").  You may
do so by creating a todo list like this one:

-------------------------------------------
pick deadbee Implement feature XXX
fixup f1a5c00 Fix to feature XXX
exec make
pick c0ffeee The oneline of the next commit
edit deadbab The oneline of the commit after
exec cd subdir; make test
...
-------------------------------------------

The interactive rebase will stop when a command fails (i.e. exits with
non-0 status) to give you an opportunity to fix the problem. You can
continue with `git rebase --continue`.

The "exec" command launches the command in a shell (the one specified
in `$SHELL`, or the default shell if `$SHELL` is not set), so you can
use shell features (like "cd", ">", ";" ...). The command is run from
the root of the working tree.

----------------------------------
$ git rebase -i --exec "make test"
----------------------------------

This command lets you check that intermediate commits are compilable.
The todo list becomes like that:

--------------------
pick 5928aea one
exec make test
pick 04d0fda two
exec make test
pick ba46169 three
exec make test
pick f4593f9 four
exec make test
--------------------

SPLITTING COMMITS
-----------------

In interactive mode, you can mark commits with the action "edit".  However,
this does not necessarily mean that 'git rebase' expects the result of this
edit to be exactly one commit.  Indeed, you can undo the commit, or you can
add other commits.  This can be used to split a commit into two:

- Start an interactive rebase with `git rebase -i <commit>^`, where
  <commit> is the commit you want to split.  In fact, any commit range
  will do, as long as it contains that commit.

- Mark the commit you want to split with the action "edit".

- When it comes to editing that commit, execute `git reset HEAD^`.  The
  effect is that the HEAD is rewound by one, and the index follows suit.
  However, the working tree stays the same.

- Now add the changes to the index that you want to have in the first
  commit.  You can use `git add` (possibly interactively) or
  'git gui' (or both) to do that.

- Commit the now-current index with whatever commit message is appropriate
  now.

- Repeat the last two steps until your working tree is clean.

- Continue the rebase with `git rebase --continue`.

If you are not absolutely sure that the intermediate revisions are
consistent (they compile, pass the testsuite, etc.) you should use
'git stash' to stash away the not-yet-committed changes
after each commit, test, and amend the commit if fixes are necessary.


RECOVERING FROM UPSTREAM REBASE
-------------------------------

Rebasing (or any other form of rewriting) a branch that others have
based work on is a bad idea: anyone downstream of it is forced to
manually fix their history.  This section explains how to do the fix
from the downstream's point of view.  The real fix, however, would be
to avoid rebasing the upstream in the first place.

To illustrate, suppose you are in a situation where someone develops a
'subsystem' branch, and you are working on a 'topic' that is dependent
on this 'subsystem'.  You might end up with a history like the
following:

------------
    o---o---o---o---o---o---o---o  master
	 \
	  o---o---o---o---o  subsystem
			   \
			    *---*---*  topic
------------

If 'subsystem' is rebased against 'master', the following happens:

------------
    o---o---o---o---o---o---o---o  master
	 \			 \
	  o---o---o---o---o	  o'--o'--o'--o'--o'  subsystem
			   \
			    *---*---*  topic
------------

If you now continue development as usual, and eventually merge 'topic'
to 'subsystem', the commits from 'subsystem' will remain duplicated forever:

------------
    o---o---o---o---o---o---o---o  master
	 \			 \
	  o---o---o---o---o	  o'--o'--o'--o'--o'--M	 subsystem
			   \			     /
			    *---*---*-..........-*--*  topic
------------

Such duplicates are generally frowned upon because they clutter up
history, making it harder to follow.  To clean things up, you need to
transplant the commits on 'topic' to the new 'subsystem' tip, i.e.,
rebase 'topic'.  This becomes a ripple effect: anyone downstream from
'topic' is forced to rebase too, and so on!

There are two kinds of fixes, discussed in the following subsections:

Easy case: The changes are literally the same.::

	This happens if the 'subsystem' rebase was a simple rebase and
	had no conflicts.

Hard case: The changes are not the same.::

	This happens if the 'subsystem' rebase had conflicts, or used
	`--interactive` to omit, edit, squash, or fixup commits; or
	if the upstream used one of `commit --amend`, `reset`, or
	`filter-branch`.


The easy case
~~~~~~~~~~~~~

Only works if the changes (patch IDs based on the diff contents) on
'subsystem' are literally the same before and after the rebase
'subsystem' did.

In that case, the fix is easy because 'git rebase' knows to skip
changes that are already present in the new upstream.  So if you say
(assuming you're on 'topic')
------------
    $ git rebase subsystem
------------
you will end up with the fixed history
------------
    o---o---o---o---o---o---o---o  master
				 \
				  o'--o'--o'--o'--o'  subsystem
						   \
						    *---*---*  topic
------------


The hard case
~~~~~~~~~~~~~

Things get more complicated if the 'subsystem' changes do not exactly
correspond to the ones before the rebase.

NOTE: While an "easy case recovery" sometimes appears to be successful
      even in the hard case, it may have unintended consequences.  For
      example, a commit that was removed via `git rebase
      --interactive` will be **resurrected**!

The idea is to manually tell 'git rebase' "where the old 'subsystem'
ended and your 'topic' began", that is, what the old merge-base
between them was.  You will have to find a way to name the last commit
of the old 'subsystem', for example:

* With the 'subsystem' reflog: after 'git fetch', the old tip of
  'subsystem' is at `subsystem@{1}`.  Subsequent fetches will
  increase the number.  (See linkgit:git-reflog[1].)

* Relative to the tip of 'topic': knowing that your 'topic' has three
  commits, the old tip of 'subsystem' must be `topic~3`.

You can then transplant the old `subsystem..topic` to the new tip by
saying (for the reflog case, and assuming you are on 'topic' already):
------------
    $ git rebase --onto subsystem subsystem@{1}
------------

The ripple effect of a "hard case" recovery is especially bad:
'everyone' downstream from 'topic' will now have to perform a "hard
case" recovery too!

REBASING MERGES
-----------------

The interactive rebase command was originally designed to handle
individual patch series. As such, it makes sense to exclude merge
commits from the todo list, as the developer may have merged the
then-current `master` while working on the branch, only to rebase
all the commits onto `master` eventually (skipping the merge
commits).

However, there are legitimate reasons why a developer may want to
recreate merge commits: to keep the branch structure (or "commit
topology") when working on multiple, inter-related branches.

In the following example, the developer works on a topic branch that
refactors the way buttons are defined, and on another topic branch
that uses that refactoring to implement a "Report a bug" button. The
output of `git log --graph --format=%s -5` may look like this:

------------
*   Merge branch 'report-a-bug'
|\
| * Add the feedback button
* | Merge branch 'refactor-button'
|\ \
| |/
| * Use the Button class for all buttons
| * Extract a generic Button class from the DownloadButton one
------------

The developer might want to rebase those commits to a newer `master`
while keeping the branch topology, for example when the first topic
branch is expected to be integrated into `master` much earlier than the
second one, say, to resolve merge conflicts with changes to the
DownloadButton class that made it into `master`.

This rebase can be performed using the `--rebase-merges` option.
It will generate a todo list looking like this:

------------
label onto

# Branch: refactor-button
reset onto
pick 123456 Extract a generic Button class from the DownloadButton one
pick 654321 Use the Button class for all buttons
label refactor-button

# Branch: report-a-bug
reset refactor-button # Use the Button class for all buttons
pick abcdef Add the feedback button
label report-a-bug

reset onto
merge -C a1b2c3 refactor-button # Merge 'refactor-button'
merge -C 6f5e4d report-a-bug # Merge 'report-a-bug'
------------

In contrast to a regular interactive rebase, there are `label`, `reset`
and `merge` commands in addition to `pick` ones.

The `label` command associates a label with the current HEAD when that
command is executed. These labels are created as worktree-local refs
(`refs/rewritten/<label>`) that will be deleted when the rebase
finishes. That way, rebase operations in multiple worktrees linked to
the same repository do not interfere with one another. If the `label`
command fails, it is rescheduled immediately, with a helpful message how
to proceed.

The `reset` command resets the HEAD, index and worktree to the specified
revision. It is isimilar to an `exec git reset --hard <label>`, but
refuses to overwrite untracked files. If the `reset` command fails, it is
rescheduled immediately, with a helpful message how to edit the todo list
(this typically happens when a `reset` command was inserted into the todo
list manually and contains a typo).

The `merge` command will merge the specified revision into whatever is
HEAD at that time. With `-C <original-commit>`, the commit message of
the specified merge commit will be used. When the `-C` is changed to
a lower-case `-c`, the message will be opened in an editor after a
successful merge so that the user can edit the message.

If a `merge` command fails for any reason other than merge conflicts (i.e.
when the merge operation did not even start), it is rescheduled immediately.

At this time, the `merge` command will *always* use the `recursive`
merge strategy, with no way to choose a different one. To work around
this, an `exec` command can be used to call `git merge` explicitly,
using the fact that the labels are worktree-local refs (the ref
`refs/rewritten/onto` would correspond to the label `onto`, for example).

Note: the first command (`label onto`) labels the revision onto which
the commits are rebased; The name `onto` is just a convention, as a nod
to the `--onto` option.

It is also possible to introduce completely new merge commits from scratch
by adding a command of the form `merge <merge-head>`. This form will
generate a tentative commit message and always open an editor to let the
user edit it. This can be useful e.g. when a topic branch turns out to
address more than a single concern and wants to be split into two or
even more topic branches. Consider this todo list:

------------
pick 192837 Switch from GNU Makefiles to CMake
pick 5a6c7e Document the switch to CMake
pick 918273 Fix detection of OpenSSL in CMake
pick afbecd http: add support for TLS v1.3
pick fdbaec Fix detection of cURL in CMake on Windows
------------

The one commit in this list that is not related to CMake may very well
have been motivated by working on fixing all those bugs introduced by
switching to CMake, but it addresses a different concern. To split this
branch into two topic branches, the todo list could be edited like this:

------------
label onto

pick afbecd http: add support for TLS v1.3
label tlsv1.3

reset onto
pick 192837 Switch from GNU Makefiles to CMake
pick 918273 Fix detection of OpenSSL in CMake
pick fdbaec Fix detection of cURL in CMake on Windows
pick 5a6c7e Document the switch to CMake
label cmake

reset onto
merge tlsv1.3
merge cmake
------------

BUGS
----
The todo list presented by `--preserve-merges --interactive` does not
represent the topology of the revision graph.  Editing commits and
rewording their commit messages should work fine, but attempts to
reorder commits tend to produce counterintuitive results. Use
`--rebase-merges` in such scenarios instead.

For example, an attempt to rearrange
------------
1 --- 2 --- 3 --- 4 --- 5
------------
to
------------
1 --- 2 --- 4 --- 3 --- 5
------------
by moving the "pick 4" line will result in the following history:
------------
	3
       /
1 --- 2 --- 4 --- 5
------------

GIT
---
Part of the linkgit:git[1] suite