Can you give me a reason for "subpoenable" as a requirement? I mean wouldn't the github data be authoritative and easy to check? Or do we really need to be forcing githubs lawyers to get involved to provide answers to a subpoena if there's a lawsuit?
Anyway, you make a great point about the commits being clear assignment of copyright, and thanks for that!
The subpoenable requirement is because you can rewrite history in git. If you just have a local copy of a repository on your hard disk, your opponent's lawyers might argue "Well, you might have used git commit --amend or manually altered the bits of the repository", and you can't show otherwise. Once you push to GitHub, the repository has become public, and a copy exists somewhere where you can't just manually alter the bits or run git commit --amend.
You don't have to actually subpoena GitHub and get their lawyers involved. The fact that you can means that opposing council will know that you're not bluffing or lying when you say "I authored this commit, and I have the commit logs to prove it". Or if they think you are bluffing or lying, they can certainly subpoena GitHub themselves...but if you aren't, that works in your favor.
Anyway, you make a great point about the commits being clear assignment of copyright, and thanks for that!