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>

Documentation/git-diff.txt

@@ -102,7 +102,11 @@ If --merge-base is given, use the merge base of the two commits for the
 Just in case you are doing something exotic, it should be
 noted that all of the <commit> in the above description, except
 in the `--merge-base` case and in the last two forms that use `..`
-notations, can be any <tree>.
+notations, can be any <tree>. A tree of interest is the one pointed to
+by the special ref `AUTO_MERGE`, which is written by the 'ort' merge
+strategy upon hitting merge conflicts (see linkgit:git-merge[1]).
+Comparing the working tree with `AUTO_MERGE` shows changes you've made
+so far to resolve textual conflicts (see the examples below).
 
 For a more complete list of ways to spell <commit>, see
 "SPECIFYING REVISIONS" section in linkgit:gitrevisions[7].
@@ -152,6 +156,7 @@ Various ways to check your working tree::
 $ git diff            <1>
 $ git diff --cached   <2>
 $ git diff HEAD       <3>
+$ git diff AUTO_MERGE <4>
 ------------
 +
 <1> Changes in the working tree not yet staged for the next commit.
@@ -159,6 +164,8 @@ $ git diff HEAD       <3>
     would be committing if you run `git commit` without `-a` option.
 <3> Changes in the working tree since your last commit; what you
     would be committing if you run `git commit -a`
+<4> Changes in the working tree you've made to resolve textual
+    conflicts so far.
 
 Comparing with arbitrary commits::
 +

Documentation/git-merge.txt

@@ -196,7 +196,11 @@ happens:
    can inspect the stages with `git ls-files -u`).  The working
    tree files contain the result of the merge operation; i.e. 3-way
    merge results with familiar conflict markers `<<<` `===` `>>>`.
-5. No other changes are made.  In particular, the local
+5. A special ref `AUTO_MERGE` is written, pointing to a tree
+   corresponding to the current content of the working tree (including
+   conflict markers for textual conflicts).  Note that this ref is only
+   written when the 'ort' merge strategy is used (the default).
+6. No other changes are made.  In particular, the local
    modifications you had before you started merge will stay the
    same and the index entries for them stay as they were,
    i.e. matching `HEAD`.
@@ -336,7 +340,8 @@ You can work through the conflict with a number of tools:
 
  * Look at the diffs.  `git diff` will show a three-way diff,
    highlighting changes from both the `HEAD` and `MERGE_HEAD`
-   versions.
+   versions. `git diff AUTO_MERGE` will show what changes you've
+   made so far to resolve textual conflicts.
 
  * Look at the diffs from each branch. `git log --merge -p <path>`
    will show diffs first for the `HEAD` version and then the

Documentation/revisions.txt

@@ -33,7 +33,8 @@ characters and to avoid word splitting.
 +
   . If '$GIT_DIR/<refname>' exists, that is what you mean (this is usually
     useful only for `HEAD`, `FETCH_HEAD`, `ORIG_HEAD`, `MERGE_HEAD`,
-    `REBASE_HEAD`, `REVERT_HEAD`, `CHERRY_PICK_HEAD` and `BISECT_HEAD`);
+    `REBASE_HEAD`, `REVERT_HEAD`, `CHERRY_PICK_HEAD`, `BISECT_HEAD`
+    and `AUTO_MERGE`);
 
   . otherwise, 'refs/<refname>' if it exists;
 
@@ -71,6 +72,10 @@ characters and to avoid word splitting.
   `BISECT_HEAD`:::
     records the current commit to be tested when you run `git bisect
     --no-checkout`.
+  `AUTO_MERGE`:::
+    records a tree object corresponding to the state the
+    'ort' merge strategy wrote to the working tree when a merge operation
+    resulted in conflicts.
 
 +
 Note that any of the 'refs/*' cases above may come either from

Documentation/user-manual.txt

@@ -1343,6 +1343,33 @@ $ git diff -3 file.txt		# diff against stage 3
 $ git diff --theirs file.txt	# same as the above.
 -------------------------------------------------
 
+When using the 'ort' merge strategy (the default), before updating the working
+tree with the result of the merge, Git writes a special ref named AUTO_MERGE
+reflecting the state of the tree it is about to write. Conflicted paths with
+textual conflicts that could not be automatically merged are written to this
+tree with conflict markers, just as in the working tree. AUTO_MERGE can thus be
+used with linkgit:git-diff[1] to show the changes you've made so far to resolve
+conflicts. Using the same example as above, after resolving the conflict we
+get:
+
+-------------------------------------------------
+$ git diff AUTO_MERGE
+diff --git a/file.txt b/file.txt
+index cd10406..8bf5ae7 100644
+--- a/file.txt
++++ b/file.txt
+@@ -1,5 +1 @@
+-<<<<<<< HEAD:file.txt
+-Hello world
+-=======
+-Goodbye
+->>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt
++Goodbye world
+-------------------------------------------------
+
+Notice that the diff shows we deleted the conflict markers and both versions of
+the content line, and wrote "Goodbye world" instead.
+
 The linkgit:git-log[1] and linkgit:gitk[1] commands also provide special help
 for merges: