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>

tree.c

@@ -197,7 +197,7 @@ int read_tree(struct tree *tree, int stage, struct pathspec *match,
 
 struct tree *lookup_tree(const struct object_id *oid)
 {
-	struct object *obj = lookup_object(oid->hash);
+	struct object *obj = lookup_object(the_repository, oid->hash);
 	if (!obj)
 		return create_object(the_repository, oid->hash,
 				     alloc_tree_node(the_repository));