Stop changing directory to the filesystem root

This behaviour dates from when Gitaly RPCs were executed in Ruby by a Go subprocess. It's not needed for gitlab-shell now that it's in Go, and it's a very strange thing for gitlab-sshd. Best just to remove it.

If we wanted to retain this behaviour, we could have an os.Chdir call in the gitlab-shell binary, but I just don't think it's needed.

Changelog: fixed

Part of #501 (closed)