When rebasing a PR in the API, refresh its cached merge_status as well
    
    Doing this in the same API call allows to specify the user who has asked
    for the rebase and thus we are able to use this information in the
    notification about the rebase in the UI, instead of relying on the name
    of the user's whose project the PR is opened against.
    
    Fixes https://pagure.io/pagure/issue/4316
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>