In the following case

  $ git rebase master
  Fast-forwarded autostash-fix to master.

The autostash is not applied automatically, because this codepath
forgets to call finish_rebase().  Fix this.  Also add a test to guard
against regressions.
Signed-off-by: NRamkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>