Undo is confused after push-line-or-edit?

[Bart Schaefer <schaefer@brasslantern.com>]

If you execute push-line-or-edit, and then any SINGLE editing action in
the multi-line buffer, and then undo, the undo reverts all the way back
to the prompt.  E.g., start with

% { : begin a block  
cursh> some more stuff 
execute: push-line-or-edit_

Which leaves you at

% { : begin a block
cursh>  
% { : begin a block
some more stuff

Now ctl-w to kill "stuff":

% { : begin a block
some more

Now ctl-x ctl-u erases everything, when I would have expected it to put
back the "stuff".  If you do more than one editing action, you can repeat
the undo back as far as the the second one, but the first one is always
skipped and the entire buffer erased instead.

Incorrect initialization of undo_changeno ?  Missing mkundoent()?