rebase: flag --apply and --merge as incompatible

rebase: flag --apply and --merge as incompatible

Previously, we flagged options which implied --apply as being
incompatible with options which implied --merge.  But if both options
were given explicitly, then we didn't flag the incompatibility.  The
same is true with --apply and --interactive.  Add the check, and add
some testcases to verify these are also caught.

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