const char *commit_type = "commit";
 
-static struct commit *check_commit(struct object *obj,
-                                  const unsigned char *sha1,
-                                  int quiet)
-{
-       if (obj->type != OBJ_COMMIT) {
-               if (!quiet)
-                       error("Object %s is a %s, not a commit",
-                             sha1_to_hex(sha1), typename(obj->type));
-               return NULL;
-       }
-       return (struct commit *) obj;
-}
-
 struct commit *lookup_commit_reference_gently(const unsigned char *sha1,
                                              int quiet)
 {
 
        if (!obj)
                return NULL;
-       return check_commit(obj, sha1, quiet);
+       return object_as_type(obj, OBJ_COMMIT, quiet);
 }
 
 struct commit *lookup_commit_reference(const unsigned char *sha1)
 struct commit *lookup_commit(const unsigned char *sha1)
 {
        struct object *obj = lookup_object(sha1);
-       if (!obj) {
-               struct commit *c = alloc_commit_node();
-               return create_object(sha1, OBJ_COMMIT, c);
-       }
-       if (!obj->type)
-               obj->type = OBJ_COMMIT;
-       return check_commit(obj, sha1, 0);
+       if (!obj)
+               return create_object(sha1, alloc_commit_node());
+       return object_as_type(obj, OBJ_COMMIT, 0);
 }
 
 struct commit *lookup_commit_reference_by_name(const char *name)
                p->item->object.flags |= STALE;
                num_head++;
        }
-       array = xcalloc(sizeof(*array), num_head);
+       array = xcalloc(num_head, sizeof(*array));
        for (p = heads, i = 0; p; p = p->next) {
                if (p->item->object.flags & STALE) {
                        array[i++] = p->item;