Normalize hashes, throw exception on things that aren't hashes

[fredemmott]

• normalize to the full commit hashes
• hg can be extended to support custom commit identifiers - subversion IDs, differential, etc. These should be banned: some are ambiguous, others may be local-only. Generally, there's a risk of users depending on behavior that is out of our control. Possible fix: assert that original hash is a substring of the normalized hash.