Amending

`git commit --amend` does not modify a commit. Commits are immutable — their id is a hash of their contents, so changing one would change its name. Amend builds a new commit from the current index and moves the branch to it.

The commit you replaced is still in the object store, unreferenced, and the reflog still names it. That is what makes an unwanted amend recoverable.

Because the id changes, amending a commit you have already pushed means the remote and you now disagree — which is the whole of the force-push problem, met early.