Look at the URL: "rewriting-history"

Ammending changes the hash of the commit. That doesn't matter if you haven't pushed it yet but if you have, you can't push the change upstream without force-pushing.

And if I want to change a commit message further back in history (regardless of my own or of somebody else), it would branch off at that point from the upstream git history.

What I want is that you could put multiple comments onto the same commit with different timestamps. Like git versions files, it should also allow to version commit messages.