revert clear-commit-marks for now.

Earlier change broke "git describe A B" among other things.
Revert it for now, and clean the commits smudged by
get_merge_bases using clear_object_marks() function.  For
complex commit ancestry graph, this is way cheaper as well.

Signed-off-by: Junio C Hamano <junkio@cox.net>

commit.c

@@ -397,12 +397,13 @@ void clear_commit_marks(struct commit *commit, unsigned int mark)
 {
 	struct commit_list *parents;
 
-	if (!commit)
-		return;
 	parents = commit->parents;
 	commit->object.flags &= ~mark;
 	while (parents) {
-		clear_commit_marks(parents->item, mark);
+		struct commit *parent = parents->item;
+		if (parent && parent->object.parsed &&
+		    (parent->object.flags & mark))
+			clear_commit_marks(parent, mark);
 		parents = parents->next;
 	}
 }
@@ -1082,10 +1083,8 @@ struct commit_list *get_merge_bases(struct commit *rev1, struct commit *rev2,
 	}
 
  finish:
-	if (cleanup) {
-		clear_commit_marks(rev1, PARENT1 | PARENT2 | STALE);
-		clear_commit_marks(rev2, PARENT1 | PARENT2 | STALE);
-	}
+	if (cleanup)
+		clear_object_marks(PARENT1 | PARENT2 | STALE);
 
 	return result;
 }

object.c

@@ -217,3 +217,12 @@ void add_object_array(struct object *obj, const char *name, struct object_array
 	objects[nr].name = name;
 	array->nr = ++nr;
 }
+
+void clear_object_marks(unsigned mark)
+{
+	int i;
+
+	for (i = 0; i < obj_allocs; i++)
+		if (objs[i])
+			objs[i]->flags &= ~mark;
+}

object.h

@@ -83,4 +83,6 @@ int object_list_contains(struct object_list *list, struct object *obj);
 /* Object array handling .. */
 void add_object_array(struct object *obj, const char *name, struct object_array *array);
 
+void clear_object_marks(unsigned);
+
 #endif /* OBJECT_H */