restage formatted files to make hook succeed first time

[IanButterworth]

On the Pkg repo if I have bad formatting, when I commit the hook will fail but actually fix the formatting, then the 2nd commit will succeed.

I think this fixes the issue, which is that we're not re-staging files after modifying them?

cc. @KristofferC

[fredrikekre]

Not sure about this, I think the existing code works the way most other similar pre-commit hooks work, i.e. you have to inspect the changes and stage them yourself.

[IanButterworth]

I see. I think the thing that's making this awkward for me is that I use GitHub Desktop, which automatically stages files you've selected in the UI when you press commit, and if the commit fails it remembers the ones you've selected. So perhaps it should be unselecting files that have been changed or something.

[IanButterworth]

Or maybe it's that the hook reports formatting "failed" rather than something like "failed: auto-formatted, review changes"