> This totally breaks a fundamental notion in Git that the same work, based on the same commits, has the same hash.

This is not a fundamental notion in git. The hash includes timestamps as well.

edit:

It also contains author and commiter details. Here's a slightly revised list borrowed from a blog post[1]

- commit message

- The file changes

- The commit author (and committer- they can be different)

- The date

- The parent commit hash(es)

[1]: https://www.mikestreety.co.uk/blog/the-git-commit-hash