This is an awesome idea.
Can't wait to hear the objections from the purists.
- The interactive interface of `git undo` is probably too hard for beginners. Designing simple interfaces is hard, and you can't pplease everyone.
- It seems `git undo` simply does a `git reset--hard` toward a designed commit. The user probably expected to return to the state before the last git action. Instead `git undo` reverted the amend, but also reset the index and remove the local changes. At this point, the beginner could scream "Undo lost my changes! I just wanted a second commit instead of an amend."
Undoing could mean `git reset --hard {hash}`, but in many cases `--hard` would have unwanted effect and the default `--mixed` would be more suitable. But even in this case, it's not a full undo, since the index is not preserved: you don't end up in the same state as before the `commit --amend`.
It runs a `git checkout`, not a `git reset --hard`, so it will stop you if there are incompatible changes. Of course, if `git undo` can be made to lose your staged or unstaged work, then that's a bug.
Fortunately, you can undo the result of any `git undo` with another `git undo` that goes one more step back in time.