Merge branch 'ml/log-merge-with-cherry-pick-and-other-pseudo-heads'

"git log --merge" learned to pay attention to CHERRY_PICK_HEAD and
other kinds of *_HEAD pseudorefs.

* ml/log-merge-with-cherry-pick-and-other-pseudo-heads:
  revision: implement `git log --merge` also for rebase/cherry-pick/revert
  revision: ensure MERGE_HEAD is a ref in prepare_show_merge

Author: gitster

Documentation/rev-list-options.txt

@@ -341,8 +341,11 @@ See also linkgit:git-reflog[1].
 Under `--pretty=reference`, this information will not be shown at all.
 
 --merge::
-	After a failed merge, show refs that touch files having a
-	conflict and don't exist on all heads to merge.
+	Show commits touching conflicted paths in the range `HEAD...<other>`,
+	where `<other>` is the first existing pseudoref in `MERGE_HEAD`,
+	`CHERRY_PICK_HEAD`, `REVERT_HEAD` or `REBASE_HEAD`. Only works
+	when the index has unmerged entries. This option can be used to show
+	relevant commits when resolving conflicts from a 3-way merge.
 
 --boundary::
 	Output excluded boundary commits. Boundary commits are

revision.c

@@ -1970,23 +1970,42 @@ static void add_pending_commit_list(struct rev_info *revs,
 	}
 }
 
+static const char *lookup_other_head(struct object_id *oid)
+{
+	int i;
+	static const char *const other_head[] = {
+		"MERGE_HEAD", "CHERRY_PICK_HEAD", "REVERT_HEAD", "REBASE_HEAD"
+	};
+
+	for (i = 0; i < ARRAY_SIZE(other_head); i++)
+		if (!read_ref_full(other_head[i],
+				RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+				oid, NULL)) {
+			if (is_null_oid(oid))
+				die(_("%s exists but is a symbolic ref"), other_head[i]);
+			return other_head[i];
+		}
+
+	die(_("--merge requires one of the pseudorefs MERGE_HEAD, CHERRY_PICK_HEAD, REVERT_HEAD or REBASE_HEAD"));
+}
+
 static void prepare_show_merge(struct rev_info *revs)
 {
 	struct commit_list *bases;
 	struct commit *head, *other;
 	struct object_id oid;
+	const char *other_name;
 	const char **prune = NULL;
 	int i, prune_num = 1; /* counting terminating NULL */
 	struct index_state *istate = revs->repo->index;
 
 	if (repo_get_oid(the_repository, "HEAD", &oid))
 		die("--merge without HEAD?");
 	head = lookup_commit_or_die(&oid, "HEAD");
-	if (repo_get_oid(the_repository, "MERGE_HEAD", &oid))
-		die("--merge without MERGE_HEAD?");
-	other = lookup_commit_or_die(&oid, "MERGE_HEAD");
+	other_name = lookup_other_head(&oid);
+	other = lookup_commit_or_die(&oid, other_name);
 	add_pending_object(revs, &head->object, "HEAD");
-	add_pending_object(revs, &other->object, "MERGE_HEAD");
+	add_pending_object(revs, &other->object, other_name);
 	bases = repo_get_merge_bases(the_repository, head, other);
 	add_rev_cmdline_list(revs, bases, REV_CMD_MERGE_BASE, UNINTERESTING | BOTTOM);
 	add_pending_commit_list(revs, bases, UNINTERESTING | BOTTOM);