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

[Eli Zaretskii]

> Date: Sat, 12 Oct 2019 20:17:02 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 37700@debbugs.gnu.org, tkk@misasa.okayama-u.ac.jp, 
> homeros.misasa@gmail.com
> 
> perhaps have a list of commands for which an immediate 'undo' will
> disregard an active region.

Or maybe have those commands have an 'undo' property which would tell
'undo' to treat them specially.  Like we do with delete-selection.