object: add clear_commit_marks_all()

Add a function for clearing the commit marks of all in-core commit
objects.  It's similar to clear_object_flags(), but more precise, since
it leaves the other object types alone.  It still has to iterate through
them, though.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object.c

@@ -434,3 +434,14 @@ void clear_object_flags(unsigned flags)
 			obj->flags &= ~flags;
 	}
 }
+
+void clear_commit_marks_all(unsigned int flags)
+{
+	int i;
+
+	for (i = 0; i < obj_hash_size; i++) {
+		struct object *obj = obj_hash[i];
+		if (obj && obj->type == OBJ_COMMIT)
+			obj->flags &= ~flags;
+	}
+}