Ask HN: Using git commit hashes in legal documents?

I was just wondering: has anyone ever used VCS version numbers / hashes in legal documents for transfer of intellectual property?

This is the scenario I had in mind: I want to get some people to help me out with some code. Contributors work on code remotely, check in their final contribution & sign off on an intellectual property transfer sheet with the exact git commit hash? Seems like an elegant solution to me, but IANAL. What's the catch?