shallow: add repository argument to is_repository_shallow

Add a repository argument to allow callers of is_repository_shallow
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>

commit.c

@@ -208,7 +208,7 @@ static void prepare_commit_graft_the_repository(void)
 	graft_file = get_graft_file();
 	read_graft_file(the_repository, graft_file);
 	/* make sure shallows are read */
-	is_repository_shallow();
+	is_repository_shallow(the_repository);
 	commit_graft_prepared = 1;
 }