builtin rebase: support `--autostash` option

builtin rebase: support `--autostash` option

To support `--autostash` we introduce a function `apply_autostash()`
just like in `git-legacy-rebase.sh`.

Rather than refactoring and using the same function that exists in
`sequencer.c`, we go a different route here, to avoid clashes with
the sister GSoC project that turns the interactive rebase into a
builtin.

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