rebase: update `--empty=ask` to `--empty=stop`

rebase: update `--empty=ask` to `--empty=stop`

When git-am(1) got its own `--empty` option in 7c096b8d61 (am: support
--empty=<option> to handle empty patches, 2021-12-09), `stop` was used
instead of `ask`. `stop` is a more accurate term for describing what
really happens, and consistency is good.

Update git-rebase(1) to also use `stop`, while keeping `ask` as a
deprecated synonym. Update the tests to primarily use `stop`, but also
ensure that `ask` is still allowed.

In a future commit, we'll be adding a new `--empty` option for
git-cherry-pick(1) as well, making the consistency even more relevant.

Reported-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Brian Lyles <brianmlyles@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>