bug#23871: 25.1.50; Undo unexpectedly leads to blank buffer

[Eli Zaretskii <eliz@gnu.org>]

> From: phillip.lord@russet.org.uk (Phillip Lord)
> Cc: triska@metalevel.at,  23871@debbugs.gnu.org
> Date: Sat, 02 Jul 2016 21:21:28 +0100
> 
> > The problem, AFAIU, is in point movements _during_ eval-defun: they
> > seem to not be recorded in buffer-undo-list.  The undo list I get
> > after C-M-x is this:
> >
> >   (nil (117 . 122) (t 22391 27551 0 0))
> >
> 
> So, I don't think this is a regression caused by my patch at all. It is
> an bug that has been there since I altered undo.c last year.
> 
> The problem was caused because of undo only records point after a
> boundary (or the first element). I'd changed things during slightly when
> I update undo.c so that the timestamp list got added before checking
> whether I was at a boundary, hence blocking addition of the point
> restoration information.
> 
> This is why I found the problem so erratic to replicate; it only occurs
> immediately the first change to a buffer.
> 
> I believe the following patch addresses the issue.

Seems like this is still not the final patch, so I guess we will need
to wait for the right one.  It should go to the emacs-25 branch, which
probably means one more pretest, sigh.

Thanks.