Fix pushing merge commits to the original repo
    
    For some reason pygit2 wasn't properly creating the local reference to
    be pushed to the main repo. With this commit we enforce that all merge
    commits use a local reference that is then pushed to the target branch.
    
    Fixes https://pagure.io/pagure/issue/1190
    Fixes https://pagure.io/pagure/issue/2119
    
    Thanks to @puiterwijk for the team work on tackling this one
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>