ci: Check that allow-collaboration is checked for merge requests

This required for merging to be possible, so make the lack of this check
box being checked a discoverable error.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1586>

.gitlab-ci.yml

@@ -83,6 +83,24 @@ check-commit-log:
     reports:
       junit: commit-message-junit-report.xml
 
+check-merge-request:
+  extends:
+    - .fdo.ci-fairy
+  stage: review
+  script:
+    - if [[ x"$CI_MERGE_REQUEST_TARGET_BRANCH_NAME" != "x" ]] ;
+      then
+        ci-fairy check-merge-request --require-allow-collaboration --junit-xml=check-merge-request-report.xml ;
+      else
+        echo "Not a merge request" ;
+      fi
+  artifacts:
+    expire_in: 1 week
+    paths:
+      - check-merge-request-report.xml
+    reports:
+      junit: check-merge-request-report.xml
+
 build-mutter:
   extends:
     - .fdo.distribution-image@fedora