Update the PR ref when the PR is updated
    
    Sources: https://git-scm.com/docs/git-push
    """
    By having the optional leading +, you can tell Git to update the
    <dst> ref even if it is not allowed by default (e.g., it is not
    a fast-forward.) This does not attempt to merge <src> into <dst>.
    [...]
    To force a push to only one branch, use a + in front of the refspec to push
    """
    
    Fixes https://pagure.io/pagure/issue/2452
    
    We are also adding unit-tests checking that the refs gets updated when
    the PR is.
    Finally, we had to adjust the unit-tests to prevent trying to push to
    non-bare repos the refs of the PR.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>