See T12039 for some context.

• We should make it more clear in the documentation that we need the upstream, unforked hashes.
• We should make version generation try harder to find an appropriate base revision.
  • List origin URIs.
  • Filter just the ones on secure.phabricator.com or github.com/phacility.
  • Pick origin if possible.
  • Pick anything else randomly if not? This could possibly be smarter but probably not necessary.
  • Report git merge-base HEAD <that origin>/master if we get this far, and it differs from HEAD.