versioneer does not fall back to parent-name-prefix method when used with "git archive" on an untagged revision

[chrivers]

I believe I've discovered a bug in versioneer. Here's an example repos:

$ git log --format=oneline --abbrev-commit --decorate
e85b611 (HEAD, master)  + Added gitignore
e19e5a6  + Added .gitattributes
d3f82ec  + Added example files
0098d7a  + Added versioneer

$ git tag -a 0.1 -m 0.1 e19e5a6

$ ./setup.py -V
0.1+1.ge85b611

$ mkdir head  &&  git archive --format=tar HEAD | tar -C head -x

$ mkdir head-parent &&  git archive --format=tar 'HEAD~1' | tar -C head-parent -x

$ cd head && ./setup.py version
running version
discarding 'HEAD,master', no digits
likely tags:
no suitable tags, using unknown + full revision id
got version from expanded keyword {'error': 'no suitable tags', 'date': None, 'version': '0+unknown', 'dirty': False, 'full-revisionid': 'e85b611532c7611077ffd2bf2eeb039d76b2e61a'}
Version: 0+unknown
 full-revisionid: e85b611532c7611077ffd2bf2eeb039d76b2e61a
 dirty: False
 date: None
 error: no suitable tags

$ cd head-parent && ./setup.py version
running version
likely tags: 0.1
picking 0.1
got version from expanded keyword {'error': None, 'date': '2016-11-16T12:50:37+0100', 'version': '0.1', 'dirty': False, 'full-revisionid': 'e19e5a611f5bb1deba1586579ce4684e95297b6a'}
Version: 0.1
 full-revisionid: e19e5a611f5bb1deba1586579ce4684e95297b6a
 dirty: False
 date: 2016-11-16T12:50:37+0100

Getting version 0+unknown isn't particularly helpful at all here. Especially not since I can get a much more reasonable version like this:

$ rm .gitattributes

$mkdir head-no-attrib &&  git archive --format=tar --worktree-attributes 'HEAD' --prefix=problem-0.1/ | tar -C head-no-attrib -x
$ cd head-no-attrib/problem-0.1 && ./setup.py version
running version
keywords are unexpanded, not using
got version from VCS {'error': None, 'date': '2016-11-16T12:52:47+0100', 'version': '0.1+1.ge85b611.dirty', 'dirty': True, 'full-revisionid': 'e85b611532c7611077ffd2bf2eeb039d76b2e61a'}
Version: 0.1+1.ge85b611.dirty
 full-revisionid: e85b611532c7611077ffd2bf2eeb039d76b2e61a
 dirty: True
 date: 2016-11-16T12:52:47+0100

There are very reasonable use cases for this, and I don't see any value in not trying the parent-dir-prefix method if the result is otherwise going to be 0+unknown anyway. Comments?