Documentation: document AUTO_MERGE

Documentation: document AUTO_MERGE

Since 5291828df8 (merge-ort: write $GIT_DIR/AUTO_MERGE whenever we hit a
conflict, 2021-03-20), when using the 'ort' merge strategy, the special
ref AUTO_MERGE is written when a merge operation results in conflicts.
This ref points to a tree recording the conflicted state of the working
tree and is very useful during conflict resolution. However, this ref is
not documented.

Add some documentation for AUTO_MERGE in git-diff(1), git-merge(1),
gitrevisions(7) and in the user manual.

In git-diff(1), mention it at the end of the description section, when
we mention that the command also accepts trees instead of commits, and
also add an invocation to the "Various ways to check your working tree"
example.

In git-merge(1), add a step to the list of things that happen "when it
is not obvious how to reconcile the changes", under the "True merge"
section. Also mention AUTO_MERGE in the "How to resolve conflicts"
section, when mentioning 'git diff'.

In gitrevisions(7), add a mention of AUTO_MERGE along with the other
special refs.

In the user manual, add a paragraph describing AUTO_MERGE to the
"Getting conflict-resolution help during a merge" section, and include
an example of a 'git diff AUTO_MERGE' invocation for the example
conflict used in that section. Note that for uniformity we do not use
backticks around AUTO_MERGE here since the rest of the document does not
typeset special refs differently.

Closes: https://github.com/gitgitgadget/git/issues/1471
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>