Allow pull changes from a different repo than the parent one
    
    This will allow opening pull-request from a fork to another fork without
    the first fork being a fork of the second, they only share the original
    parent.
    
    This could also work for people wanting to open a pull-request against
    multiple forks of the same repo.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>