That's a good idea, but only provides full protection if all git clients do it. The official git client itself doesn't support that, so people would still be vu

That's a good idea, but only provides full protection if all git clients do it. The official git client itself doesn't support that, so people would still be vulnerable if they got a malicious half from github, or a signature on a non-malicious half from github and a malicious half somewhere else. This protection github is implementing uses a heuristic to try to find collision attempts by just looking at a single file. So it prevents github from hosting any collision attempts.