diff-merges: make -m/-c/--cc explicitly mutually exclusive

diff-merges: make -m/-c/--cc explicitly mutually exclusive

-c/--cc got precedence over -m only because of external logic where
corresponding flags are checked before that for -m. This is too
error-prone, so add code that explicitly makes these 3 options
mutually exclusive, so that the last option specified on the
command-line gets precedence.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>