git shell wrapper input verification

[madwort]

I've been eyeing this script quite suspiciously this morning:

https://github.com/opensafely/git-server-proxy/blob/master/setup/git-shell-wrapper.py#L11-L24

I didn't manage to exploit it yet, because of the multiple constraints that it would have to satisfy, but I would feel more comfortable with some explicit verification of the contents of argv before passing them to subprocess.