bug#37700: 27.0.50; undo mouse-drag-and-drop-region ineffective

[Mattias Engdegård]

Undoing mouse-drag-and-drop-region doesn't work:

1. (setq mouse-drag-and-drop-region t)
2. mark a region
3. use the mouse to drag the region elsewhere
4. Undo (twice)

Result: the dragged text disappears from its destination, but does not reappear 
where it was dragged from.
Instead, we get the "No further undo information for region" error.

The action can be undone by moving point and backing up a different undo branch 
a few steps, but that is hardly ideal.