tests (rebase): spell out the `--keep-empty` option

This test wants to run `git rebase` with the `--keep-empty` option, but
it really only spelled out `--keep` and trusted Git's option parsing to
determine that this was a unique abbreviation of the real option.

However, Denton Liu contributed a patch series in
https://public-inbox.org/git/cover.1553354374.git.liu.denton@gmail.com/
that introduces a new `git rebase` option called `--keep-base`, which
makes this previously unique abbreviation non-unique.

Whether this patch series is accepted or not, it is actually a bad
practice to use abbreviated options in our test suite, because of the
issue that those unique option names are not guaranteed to stay unique
in the future.

So let's just not use abbreviated options in the test suite.

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

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