rebase: save autostash entry into stash reflog on --quit

rebase: save autostash entry into stash reflog on --quit

In a03b55530a (merge: teach --autostash option, 2020-04-07), the
--autostash option was introduced for `git merge`. Notably, when
`git merge --quit` is run with an autostash entry present, it is saved
into the stash reflog. This is contrasted with the current behaviour of
`git rebase --quit` where the autostash entry is simply just dropped out
of existence.

Adopt the behaviour of `git merge --quit` in `git rebase --quit` and
save the autostash entry into the stash reflog instead of just deleting
it.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>