ci-fairy needs to check closed merged requests after a merge

ci-fairy check-merge-request goes through open MRs only and complains if it doesn't find one that matched the git sha. That's fine while the MR is alive but it fails for the pipeline that is run after the MR is being merged - the MR is already closed by the time ci-fairy gets to it.

Can be worked around for now using except to skip the job:

check merge request job:
  # ... ci-fairy calls etc. ...
  except:
    - master@libinput/libinput

This needs some extra logic in ci-fairy to compare the git sha for closed MRs as well.