ci: Deny a plain "Closes: #123" message in the commit message

We need the full link so let's warn contributors about this instead of
having reviewers pointing this out.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3465>

.gitlab-ci/commit-rules.yml

@@ -12,3 +12,5 @@ patterns:
     - regex: '([^.]\.|[:,;])\s*$'
       message: Commit message subject should not end with punctuation
       where: subject
+    - regex: '^Closes:?\s*#[0-9]+'
+      message: Closes instructions must be a full link