Comment 3 for bug 1374677

So this should only happen if the following is true:
 - its an exported tree (no git history)
 - no egg info is present (which sdist injects for us)

The blog post suggests Adam reached the same conclusion, so I'm not sure that this is a bug: without access to either git history, or explicit version metadata, pbr can't do its job as defined, and an error is appropriate.