subtree: fix split processing with multiple subtrees present

subtree: fix split processing with multiple subtrees present

When there are multiple subtrees present in a repository and they are
all using 'git subtree split', the 'split' command can take a
significant (and constantly growing) amount of time to run even when
using the '--rejoin' flag. This is due to the fact that when processing
commits to determine the last known split to start from when looking
for changes, if there has been a split/merge done from another subtree
there will be 2 split commits, one mainline and one subtree, for the
second subtree that are part of the processing. The non-mainline
subtree split commit will cause the processing to always need to search
the entire history of the given subtree as part of its processing even
though those commits are totally irrelevant to the current subtree
split being run.

To see this in practice you can use the open source GitHub repo
'apollo-ios-dev' and do the following in order:

-Make a changes to a file in 'apollo-ios' and 'apollo-ios-codegen'
 directories
-Create a commit containing these changes
-Do a split on apollo-ios-codegen
   - Do a fetch on the subtree repo
      - git fetch git@github.com:apollographql/apollo-ios-codegen.git
   - git subtree split --prefix=apollo-ios-codegen --squash --rejoin
   - Depending on the current state of the 'apollo-ios-dev' repo
     you may see the issue at this point if the last split was on
     apollo-ios
-Do a split on apollo-ios
   - Do a fetch on the subtree repo
      - git fetch git@github.com:apollographql/apollo-ios.git
   - git subtree split --prefix=apollo-ios --squash --rejoin
-Make changes to a file in apollo-ios-codegen
-Create a commit containing the change(s)
-Do a split on apollo-ios-codegen
   - git subtree split --prefix=apollo-ios-codegen --squash --rejoin
-To see that the patch fixes the issue you can use the custom subtree
 script in the repo so following the same steps as above, except
 instead of using 'git subtree ...' for the commands use
 'git-subtree.sh ...' for the commands

You will see that the final split is looking for the last split
on apollo-ios-codegen to use as it's starting point to process
commits. Since there is a split commit from apollo-ios in between the
2 splits run on apollo-ios-codegen, the processing ends up traversing
the entire history of apollo-ios which increases the time it takes to
do a split based on how long of a history apollo-ios has, while none
of these commits are relevant to the split being done on
apollo-ios-codegen.

So this commit makes a change to the processing of commits for the
split command in order to ignore non-mainline commits from other
subtrees such as apollo-ios in the above breakdown by adding a new
function 'should_ignore_subtree_commit' which is called during
'process_split_commit'. This allows the split/rejoin processing to
still function as expected but removes all of the unnecessary
processing that takes place currently which greatly inflates the
processing time. In the above example, previously the final split
would take ~10-12 minutes, while after this fix it takes seconds.

Added a test to validate that the proposed fix
solves the issue.

The test accomplishes this by checking the output
of the split command to ensure the output from
the progress of 'process_split_commit' function
that represents the 'extracount' of commits
processed remains at 0, meaning none of the commits
from the second subtree were processed.

This was tested against the original functionality
to show the test failed, and then with this fix
to show the test passes.

This illustrated that when using multiple subtrees,
A and B, when doing a split on subtree B, the
processing does not traverse the entire history
of subtree A which is unnecessary and would cause
the 'extracount' of processed commits to climb
based on the number of commits in the history of
subtree A.

Signed-off-by: Zach FettersMoore <zach.fetters@apollographql.com>
Reviewed-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>