Bail on merge a PR that is already closed
    
    It happened at least once that two persons requested to merge a PR at
    the same time, leading to two comments being added to the PR about it
    being merge (by each individual) while the task for the second person
    actually failed (because there was nothing to change).
    
    With this change, we're waiting for the last moment and checking if the
    PR has been merged/closed and if so, we bail.
    
    Fixes https://pagure.io/pagure/issue/2686