Create references for pull-request in the git repo for local checkout
    
    This will allow checking out locally a PR opened against a given project
    without having to add and manage a new remote.
    
    One note: while existing clone will not get the objects coming from the
    fork, new clones will download them, so git show will find the commits
    coming from forks even though they are not merged and even if the user
    did not fetch the corresponding pull/ references.
    
    Fixes https://pagure.io/pagure/issue/443
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>