Documented .git_archival.txt is not reproducible

[LecrisUT]

Specifically:

ref-names: $Format:%D$

Produces different git archive after a commit is added

< ref-names: HEAD -> master, tag: 1.30
---
> ref-names: tag: 1.30

This is rather disastrous for packagers expecting this to be stable.

Either ref-names should be eliminated, or the $Format:%D$ could be altered to eliminate any HEAD references

[RonnyPfannschmidt]

Closing as duplicate of #806