legacy-rebase: backport -C<n> and --whitespace=<option> checks

legacy-rebase: backport -C<n> and --whitespace=<option> checks

Since 04519d720114 (rebase: validate -C<n> and --whitespace=<mode>
parameters early, 2018-11-14), the built-in rebase validates the -C and
--whitespace arguments early. As this commit also introduced a
regression test for this, and as a later commit introduced the
GIT_TEST_REBASE_USE_BUILTIN mode to run tests, we now have a
"regression" in the scripted version of `git rebase` on our hands.

Backport the validation to fix this.

Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>