make-rhat-patches: Use shallow clone
    
    The selinux-policy repos are quite big - use --depth=1 to fetch only the
    latest commit of the requested branch, to save network traffic and time.
    
    A possible downside of this is that one can no longer pass a commit ID
    via REPO_SELINUX_POLICY_*BRANCH, but that's unlikely to be useful in
    practice.
    
    Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>