dim: Refuse to push with leftover dim extract-tags marker

Refuse the push if there are leftover markers from
dim extract-tags still present in the commit messages.

A few of these slipped through to drm-misc-next. Let's try
to avoid that in the future.

Cc: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>