Warn users when a project does not support direct push
    
    Projects enforcing contributions via pull-request do not support direct
    push to their git repo (which is reflected in the gitolite.conf ACLs for
    this project).
    
    Fixes https://pagure.io/pagure/issue/2773
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>