But is "commit" in this context a full snapshot or a delta? Because what you describe will only work for the former.

Regardless, commit hashes should be a security mechanism IMO. And not just commits. I should be able to treat _any_ content addressing system as having secure addresses. If you can engineer collisions you need to patch your system.

(Note that the above does not necessarily imply support for unconditionally forcing a fork of the entire git ecosystem.)