ci: move merge-request check into separate stage

The current merge-request job is pretty suboptimal for non-MR branches:

In the gitlab pipeline view, non-MR branches are always shown with warning,
since this check always fails if the branch isn't associated with some MR.
Thus one always has to check the details page in order to see whether there
was something else in that stage (eg. unclean commit messages, etc) or just
the always failing merge-request check.

A trivial solution to that problem is just moving that check into a fourth
stage, so it's trivial to differenciate in the pipeline list view.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1335>

.gitlab-ci.yml

@@ -34,6 +34,7 @@ stages:
     - docker-image
     - build-and-test
     - test
+    - postcheck
 
 .ci-run-policy:
   # Retry jobs after runner system failures
@@ -131,7 +132,7 @@ check-commits:
 check-merge-request:
   extends:
     - .fdo.ci-fairy
-  stage: test
+  stage: postcheck
   script:
     - ci-fairy check-merge-request --require-allow-collaboration --junit-xml=results.xml
   artifacts: