Make the mirroring feature work with older git
    
    The GIT_SSH_COMMAND environment variable was added in 2.10 but we
    want pagure to work with older git version, so we are switching
    to GIT_SSH with a helper script generated on the fly.
    
    In addition, this commit adds the logic to pull all the branches stored
    in pagure so when we push --mirror they are all synced.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>