rebase: ignore failures from "gc --auto"

rebase: ignore failures from "gc --auto"

After rebasing, we call "gc --auto" to clean up if we
created a lot of loose objects. However, we do so inside an
&&-chain. If "gc --auto" fails (e.g., because a previous
background gc blocked us by leaving "gc.log" in place),
then:

  1. We will fail to clean up the state directory, leaving
     the user stuck in the rebase forever (even "git am
     --abort" doesn't work, because it calls "gc --auto"!).

  2. In some cases, we may return a bogus exit code from
     rebase, indicating failure when everything except the
     auto-gc succeeded.

We can fix this by ignoring the exit code of "gc --auto".

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