rebase: add coverage of other incompatible options
The git-rebase manual noted several sets of incompatible options, but
we were missing tests for a few of these. Further, we were missing
code checks for one of these, which could result in command line
options being silently ignored.
Also, note that adding a check for autosquash means that using
--whitespace=fix together with the config setting rebase.autosquash=true
will trigger an error. A subsequent commit will improve the error
message.
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>