rebase: clarify the OPT_CMDMODE incompatibilities

rebase: clarify the OPT_CMDMODE incompatibilities

--edit-todo was documented as being incompatible with any of the options
for the apply backend.  However, it is also incompatible with any of the
options for the merge backend, and is incompatible with any options that
are not backend specific as well.  The same can be said for --continue,
--skip, --abort, --quit, etc.

This is already somewhat implicitly covered by the synopsis, but since
"[<options>]" in the first two variants are vague it might be easy to
miss this.  That might not be a big deal, but since the rebase manpage
has to spend so much verbiage about incompatibility of options, making
a separate section for these options that are incompatible with
everything else seems clearer.  Do that, and remove the needless
inclusion of --edit-todo in the explicit incompatibility list.

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