rebase -i: restore autostash on abort

rebase -i: restore autostash on abort

When we abort an interactive rebase we do so by calling
`die_abort`, which cleans up after us by removing the rebase
state directory. If the user has requested to use the autostash
feature, though, the state directory may also contain a reference
to the autostash, which will now be deleted.

Fix the issue by trying to re-apply the autostash in `die_abort`.
This will also handle the case where the autostash does not apply
cleanly anymore by recording it in a user-visible stash.

Reported-by: Daniel Hahler <git@thequod.de>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>