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

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Fri, 11 Oct 2019 13:51:38 +0200
> 
> 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.

Why is that a problem?  Emacs cannot possibly "un-dnd" text, it can
only remove the copy, which AFAIU it does.