This adds a pipeline stage for merge requests that checks that the
commit message contains an URL to either a issue or a merge request.
This means that for merge requests without corresponding issues will
always fail initially, as the merge request URL is not known until after
it is created. This is still arguably better than accidentally merging
merge requests without URLs.
Taken from https://gitlab.gnome.org/GNOME/mutter/merge_requests/440.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/410
