object: add repository argument to lookup_object

Add a repository argument to allow callers of lookup_object to be more
specific about which repository to handle. This is a small mechanical
change; it doesn't change the implementation to handle repositories
other than the_repository yet.

As with the previous commits, use a macro to catch callers passing a
repository other than the_repository at compile time.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/unpack-objects.c

@@ -331,7 +331,7 @@ static int resolve_against_held(unsigned nr, const struct object_id *base,
 {
 	struct object *obj;
 	struct obj_buffer *obj_buffer;
-	obj = lookup_object(base->hash);
+	obj = lookup_object(the_repository, base->hash);
 	if (!obj)
 		return 0;
 	obj_buffer = lookup_object_buffer(obj);