object: convert create_object() to use object_id

There are no callers left of create_object() that aren't just passing us
the "hash" member of a "struct object_id". Let's take the whole struct,
which gets us closer to removing all raw sha1 variables.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -59,8 +59,7 @@ struct commit *lookup_commit(struct repository *r, const struct object_id *oid)
 {
 	struct object *obj = lookup_object(r, oid);
 	if (!obj)
-		return create_object(r, oid->hash,
-				     alloc_commit_node(r));
+		return create_object(r, oid, alloc_commit_node(r));
 	return object_as_type(r, obj, OBJ_COMMIT, 0);
 }