Add a new API endpoint to retrieve the list of files changed in a PR
    
    This new API endpoint allows to retrieve the list of files changed as
    well as the number of lines added/removed and whether the file was added
    removed, modified or renamed (cf the status field).
    
    Fixes https://pagure.io/pagure/issue/3686
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>