commit: add repository argument to lookup_commit

Add a repository argument to allow callers of lookup_commit 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>

object.c

@@ -212,7 +212,7 @@ struct object *parse_object_buffer_the_repository(const struct object_id *oid, e
 			}
 		}
 	} else if (type == OBJ_COMMIT) {
-		struct commit *commit = lookup_commit(oid);
+		struct commit *commit = lookup_commit(the_repository, oid);
 		if (commit) {
 			if (parse_commit_buffer(commit, buffer, size, 1))
 				return NULL;