merge-ort: write $GIT_DIR/AUTO_MERGE whenever we hit a conflict

There are a variety of questions users might ask while resolving
conflicts:
  * What changes have been made since the previous (first) parent?
  * What changes are staged?
  * What is still unstaged? (or what is still conflicted?)
  * What changes did I make to resolve conflicts so far?
The first three of these have simple answers:
  * git diff HEAD
  * git diff --cached
  * git diff
There was no way to answer the final question previously.  Adding one
is trivial in merge-ort, since it works by creating a tree representing
what should be written to the working copy complete with conflict
markers.  Simply write that tree to .git/AUTO_MERGE, allowing users to
answer the fourth question with
  * git diff AUTO_MERGE

I avoided using a name like "MERGE_AUTO", because that would be
merge-specific (much like MERGE_HEAD, REBASE_HEAD, REVERT_HEAD,
CHERRY_PICK_HEAD) and I wanted a name that didn't change depending on
which type of operation the merge was part of.

Ensure that paths which clean out other temporary operation-specific
files (e.g. CHERRY_PICK_HEAD, MERGE_MSG, rebase-merge/ state directory)
also clean out this AUTO_MERGE file.

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -737,6 +737,7 @@ static int finish_rebase(struct rebase_options *opts)
 	int ret = 0;
 
 	delete_ref(NULL, "REBASE_HEAD", NULL, REF_NO_DEREF);
+	unlink(git_path_auto_merge(the_repository));
 	apply_autostash(state_dir_path("autostash", opts));
 	close_object_store(the_repository->objects);
 	/*