Make .git_archival.txt more reproducible

[LecrisUT]

I believe the harm of ref-names outweigh the benefits of it, especially since this is mostly dependent on the git host provider that would produce the git archive.

Closes #806

[RonnyPfannschmidt]

thanks, its unfortunate as this also looses the branch name (which is sometimes needed)
but the branch name was only ever included in the branch head to begin with