rebase: fix incompatiblity checks for --[no-]reapply-cherry-picks

rebase: fix incompatiblity checks for --[no-]reapply-cherry-picks

--[no-]reapply-cherry-picks was traditionally only supported by the
sequencer.  Support was added for the apply backend, when --keep-base is
also specified, in commit ce5238a690 ("rebase --keep-base: imply
--reapply-cherry-picks", 2022-10-17).  Make the code error out when
--[no-]reapply-cherry-picks is specified AND the apply backend is used
AND --keep-base is not specified.  Also, clarify a number of comments
surrounding the interaction of these flags.

Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>