repobench is required to run in the same directory as reposurgeon git repository

Due to the git describe at: https://gitlab.com/esr/reposurgeon/-/blob/master/repobench#L54

At the very least this should probably be rev="$(cd $(dirname "${0}"); git describe HEAD)", but it's probably not a good idea to assume there's a git repo at all. Isn't repobench now packaged in the debian as well? Certainly won't be a git repo there :)