attic/_version.py from github releases change over time

[bjornfor]

Diff of two attic-0.15 source trees fetched at different times:

  $ diff -uNr /nix/store/h600g290a0h8szjhdfvfn4x7janrhd8n-attic-0.15-src/ attic-0.15-today/
  diff -uNr /nix/store/h600g290a0h8szjhdfvfn4x7janrhd8n-attic-0.15-src/attic/_version.py attic-0.15-today/attic/_version.py
  --- /nix/store/h600g290a0h8szjhdfvfn4x7janrhd8n-attic-0.15-src/attic/_version.py  1970-01-01 01:00:01.000000000 +0100
  +++ attic-0.15-today/attic/_version.py    2015-04-15 21:49:03.000000000 +0200
  @@ -10,7 +10,7 @@
   # versioneer-0.7+ (https://github.com/warner/python-versioneer)

   # these strings will be replaced by git during git-archive
  -git_refnames = " (HEAD, tag: 0.15, master)"
  +git_refnames = " (tag: 0.15)"
   git_full = "e815d6da8e15be8ccd09b7a183bb22bad25cf3e6"

Changes such as these cause extra work (however small) for downstream packagers:

NixOS/nixpkgs@e1407a3

I think this is partly a 'versioneer' and partly an 'attic' issue. I wonder if it would be deterministic if attic started using tags with a known prefix and then passed that prefix to versioneer, for instance tag_prefix = "v".