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 <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git