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>

rerere.c

diff --git a/rerere.c b/rerere.c
index d55aa8a..fd65a93 100644
--- a/rerere.c
+++ b/rerere.c
@@ -667,6 +667,8 @@
 		return error("Could not read index");
 
 	fd = setup_rerere(&merge_rr, RERERE_NOAUTOUPDATE);
+	if (fd < 0)
+		return 0;
 
 	unmerge_cache(pathspec);
 	find_conflict(&conflict);