Running on the working tree is mostly okay - just `exit 1` if changes were made and allow the user to stage+commit new changes. It isn't perfect but it doesn't require checking out a new tree.
What if I've already fixed the format issue (but not staged it). The pre-commit hook will pass, but it's not doing what the author intended (preventing unformated code from being committed).
What if I've only staged one part of a file, but the pre-commit hook fails on the unstaged portions, which should be fine since I'm not commiting or pushing those changes.
