usage: allow dying without writing an error message

Sometimes code wants to die in a situation where it already has written
an error message. To use the same error code as `die()` we have to use
`exit(128)`, which is easy to get wrong and leaves magic numbers all
over our codebase.

Teach `die_message_builtin()` to not print any error when passed a
`NULL` pointer as error string. Like this, such users can now call
`die(NULL)` to achieve the same result without any hardcoded error
codes.

Adapt a couple of builtins to use this new pattern to demonstrate that
there is a need for such a helper.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -992,7 +992,7 @@ static int update_local_ref(struct ref *ref,
 		fast_forward = repo_in_merge_bases(the_repository, current,
 						   updated);
 		if (fast_forward < 0)
-			exit(128);
+			die(NULL);
 		forced_updates_ms += (getnanotime() - t_before) / 1000000;
 	} else {
 		fast_forward = 1;