Really fix mutter checkout on stable branches
Follow-up to !509 (merged). This time it occurred to me to test with pseudo-stable branches first, so this one actually works.
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.