dim: force merge commit for backmerges

We have all these reminders to actually type a commit message, but
--no-commit doesn't imply --no-ff, so let's fix that.

Noticed because Maarten managed to do a fast-forward backmerge into
drm-misc-next somehow.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>