ci: Handle merge requests for non-master branches

If we don't find a branch that matches the branch used in the merge
request, we currently fall back to the non-merge-request matching,
i.e. first try the current shell branch, then fall back to master.

This should work for commits to upstream branches, but not for merge
requests to a stable branch. For those, the target branch name is
a better fallback.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/509

.gitlab-ci/checkout-mutter.sh

@@ -19,6 +19,8 @@ if [ "$CI_MERGE_REQUEST_TARGET_BRANCH_NAME" ]; then
   echo Looking for $merge_request_branch on remote ...
   if git fetch -q $merge_request_remote $merge_request_branch; then
     mutter_target=FETCH_HEAD
+  else
+    mutter_target=origin/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME
   fi
 fi