FileUndoManager: don't overwrite files on undo
- You write a small idea in thesis.txt
- You realize this should be called ideas.txt so you rename the file
- You write your entire thesis into thesis.txt
- You press Undo accidentally, the file gets renamed back with Overwrite enabled and lose your thesis (!)
I traced the overwrite flag all the way back to Simon's very first implementation of the class, at a time I still had hair, so this is not reverting an intentional bugfix.
Discovered when testing the error handling in FileUndoManager.