Ah, never mind, I can reproduce it on my machine, using the zipfile downloaded from https://github.com/tahoe-lafs/tahoe-lafs/archive/master.zip.
The first line of tahoe --version-and-path also gives an unknown version with the correct path (e.g. allmydata-tahoe: unknown [unknown: unknown] ([...]/tahoe-lafs-master/src)). src/allmydata/_version.py does not exist.
It appears that the github download .zip does not contain the .git directory, so I don't think this is fixable. We'll just have to avoid telling people to use that download method (or take into account that they'll get a Tahoe build that doesn't know its version).
