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>

branch.c

@@ -344,6 +344,7 @@ void remove_merge_branch_state(struct repository *r)
 	unlink(git_path_merge_rr(r));
 	unlink(git_path_merge_msg(r));
 	unlink(git_path_merge_mode(r));
+	unlink(git_path_auto_merge(r));
 	save_autostash(git_path_merge_autostash(r));
 }
 

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);
 	/*

merge-ort.c

@@ -3496,6 +3496,9 @@ void merge_switch_to_result(struct merge_options *opt,
 {
 	assert(opt->priv == NULL);
 	if (result->clean >= 0 && update_worktree_and_index) {
+		const char *filename;
+		FILE *fp;
+
 		trace2_region_enter("merge", "checkout", opt->repo);
 		if (checkout(opt, head, result->tree)) {
 			/* failure to function */
@@ -3514,6 +3517,13 @@ void merge_switch_to_result(struct merge_options *opt,
 		}
 		opt->priv = NULL;
 		trace2_region_leave("merge", "record_conflicted", opt->repo);
+
+		trace2_region_enter("merge", "write_auto_merge", opt->repo);
+		filename = git_path_auto_merge(opt->repo);
+		fp = xfopen(filename, "w");
+		fprintf(fp, "%s\n", oid_to_hex(&result->tree->object.oid));
+		fclose(fp);
+		trace2_region_leave("merge", "write_auto_merge", opt->repo);
 	}
 
 	if (display_update_msgs) {

path.c

@@ -1534,5 +1534,6 @@ REPO_GIT_PATH_FUNC(merge_rr, "MERGE_RR")
 REPO_GIT_PATH_FUNC(merge_mode, "MERGE_MODE")
 REPO_GIT_PATH_FUNC(merge_head, "MERGE_HEAD")
 REPO_GIT_PATH_FUNC(merge_autostash, "MERGE_AUTOSTASH")
+REPO_GIT_PATH_FUNC(auto_merge, "AUTO_MERGE")
 REPO_GIT_PATH_FUNC(fetch_head, "FETCH_HEAD")
 REPO_GIT_PATH_FUNC(shallow, "shallow")

path.h

@@ -176,6 +176,7 @@ struct path_cache {
 	const char *merge_mode;
 	const char *merge_head;
 	const char *merge_autostash;
+	const char *auto_merge;
 	const char *fetch_head;
 	const char *shallow;
 };
@@ -191,6 +192,7 @@ const char *git_path_merge_rr(struct repository *r);
 const char *git_path_merge_mode(struct repository *r);
 const char *git_path_merge_head(struct repository *r);
 const char *git_path_merge_autostash(struct repository *r);
+const char *git_path_auto_merge(struct repository *r);
 const char *git_path_fetch_head(struct repository *r);
 const char *git_path_shallow(struct repository *r);
 

sequencer.c

@@ -2096,6 +2096,7 @@ static int do_pick_commit(struct repository *r,
 		refs_delete_ref(get_main_ref_store(r), "", "CHERRY_PICK_HEAD",
 				NULL, 0);
 		unlink(git_path_merge_msg(r));
+		unlink(git_path_auto_merge(r));
 		fprintf(stderr,
 			_("dropping %s %s -- patch contents already upstream\n"),
 			oid_to_hex(&commit->object.oid), msg.subject);
@@ -2451,6 +2452,8 @@ void sequencer_post_commit_cleanup(struct repository *r, int verbose)
 		need_cleanup = 1;
 	}
 
+	unlink(git_path_auto_merge(r));
+
 	if (!need_cleanup)
 		return;
 
@@ -4111,6 +4114,7 @@ static int pick_commits(struct repository *r,
 			unlink(rebase_path_stopped_sha());
 			unlink(rebase_path_amend());
 			unlink(git_path_merge_head(r));
+			unlink(git_path_auto_merge(r));
 			delete_ref(NULL, "REBASE_HEAD", NULL, REF_NO_DEREF);
 
 			if (item->command == TODO_BREAK) {
@@ -4505,6 +4509,7 @@ static int commit_staged_changes(struct repository *r,
 		return error(_("could not commit staged changes."));
 	unlink(rebase_path_amend());
 	unlink(git_path_merge_head(r));
+	unlink(git_path_auto_merge(r));
 	if (final_fixup) {
 		unlink(rebase_path_fixup_msg());
 		unlink(rebase_path_squash_msg());