builtin rebase: support `keep-empty` option

builtin rebase: support `keep-empty` option

The `--keep-empty` option can be used to keep the commits that do not
change anything from its parents in the result.

While the scripted version uses `interactive_rebase=implied` to indicate
that the rebase needs to use the `git-rebase--interactive` backend in
non-interactive mode as fallback when figuring out which backend to use,
the C version needs to use a different route because the backend will
already be chosen during the `parse_options()` call.

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>