Update gitolite's config for the project when set to PR only
    
    Setting a project to be PR only, blocks all direct push access to it but
    for this to take effect, gitolite's config needs to be updated.
    This commit fixes this.
    
    Fixes https://pagure.io/pagure/issue/2774
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>