I still worry about just quoting things when passing it off to "ssh" or "sh -c", so I'm being anal. But _, ^ and , are certainly ok and while both ~ and @ can have speacial meaning to shell/ssh they are benign.
git-clone-pack will want it too. Soon.
It's a generic thing for matching refs from the other side.
I want to use it for git-fetch-pack too.
