rerere: exit silently on "forget" when rerere is disabled

rerere: exit silently on "forget" when rerere is disabled

If you run "git rerere forget foo" in a repository that does
not have rerere enabled, git hits an internal error:

  $ git init -q
  $ git rerere forget foo
  fatal: BUG: attempt to commit unlocked object

The problem is that setup_rerere() will not actually take
the lock if the rerere system is disabled. We should notice
this and return early. We can return with a success code
here, because we know there is nothing to forget.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>