ci: Try harder to find a matching mutter branch

Depending on how gitlab's CI checks out gnome-shell, the shell branch
may not have a local reference like "gnome-3-32", but only a remote
one like "remotes/origin/gnome-3-32".

Consider that case as well when looking for a corresponding mutter branch.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/511

.gitlab-ci/checkout-mutter.sh

@@ -26,6 +26,7 @@ fi
 
 if [ -z "$mutter_target" ]; then
   mutter_target=$(git branch -r -l origin/$shell_branch)
+  mutter_target=${mutter_target:-$(git branch -r -l ${shell_branch#remotes/})}
   mutter_target=${mutter_target:-origin/master}
   echo Using $mutter_target instead
 fi