replace-object: add repository argument to lookup_replace_object

Add a repository argument to allow callers of lookup_replace_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: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object.c

@@ -247,7 +247,7 @@ struct object *parse_object(const struct object_id *oid)
 	unsigned long size;
 	enum object_type type;
 	int eaten;
-	const struct object_id *repl = lookup_replace_object(oid);
+	const struct object_id *repl = lookup_replace_object(the_repository, oid);
 	void *buffer;
 	struct object *obj;