Re-enables format on save for agent changes (when the user has that
enabled in settings), except differently from before:
- Now we do the format-on-save in the separate buffer the edit tool
uses, *before* the diff
- This means it never triggers separate staleness
- It has the downside that edits are now blocked on the formatter
completing, but that's true of saving in general.

Release Notes:

- N/A

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Co-authored-by: Agus Zubiaga <hi@aguz.me>