sequencer (rebase -i): respect the rebase.autostash setting

sequencer (rebase -i): respect the rebase.autostash setting

Git's `rebase` command inspects the `rebase.autostash` config setting
to determine whether it should stash any uncommitted changes before
rebasing and re-apply them afterwards.

As we introduce more bits and pieces to let the sequencer act as
interactive rebase's backend, here is the part that adds support for
the autostash feature.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>