Enforce that remote PR rely on a remote git repository
    
    Otherwise, potentially, this could lead to leaking out private information
    if someone manages to open a remote PR from a private project stored of
    this pagure instance.
    
    This commit fixes the CVE: CVE-2018-1002158
    Thanks to Patrick Uiterwijk for reporting it!
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>