Fix blocking a new PR from being created if commits aren't signed-off-by
    
    If a project enforces that all of its commits must be signed-off-by,
    there is no reason to allow opening a new PR that doesn't respect this
    rule.
    We had a check for this but it looks like it was flawed. This fixes it.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>