Get the default branch of the target repo when linking for new PR
    
    Rather than assuming the default branch is the same in the source
    and the target repo, we generate the new PR url using the default
    branch of the target repo.
    
    Fixes https://pagure.io/pagure/issue/5086
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>