this has several advantages compared to an undo/redo stack private to fakevim:
- change marks in document correspond to actual changes
- mouse pasting does not invalidate the fakevim undo/redo stack
- menu commands (such as undo/redo from the Edit menu) do not invalidate the fakevim undo/redo stack
- possible other changes not caused by fakevim do not invalidate fakevim undo/redo stack

it is pretty bad if the fakevim undo/redo stack becomes invalid, as applying these invalid
undo/redo operations corrupts the document