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

[Eli Zaretskii <eliz@gnu.org>]

merge 1095 21722
thanks

> From: Markus Triska <triska@metalevel.at>
> Cc: Eli Zaretskii <eliz@gnu.org>,  23871@debbugs.gnu.org
> Date: Sun, 03 Jul 2016 12:08:01 +0200
> 
> Please note that #23871 is the main problem for me and has been present
> in Emacs for about a decade. The other issues I filed are only my
> attempts to make the issue more easily reproducible, and may in fact
> even be unrelated issues that I find while trying to find the root cause
> of #23871. I first filed #1095, then adapted this to #21722, and now
> also filed #23871 because redisplay may also be involved.

AFAICS, 1095 and 21722 report exactly the same problem; 21722 should
have never been opened as a separate bug.  So I just merged them
together.

> I have tolerated the problems described in #1095 and #21722 for about a
> decade and see no reason to delay Emacs 25: Neither Emacs 23 nor 24 were
> delayed because of them although these issues were already reported!
> #23871 was also present during this time but not yet reported.

That may be so, but my problem, which _is_ a regression from 24.5, is
that even this part of 1095:

  In "emacs -Q", when I insert into *scratch* the form:

     (progn (end-of-line) (insert "\nhi"))

  then place point on the "g" and press C-M-x C-/, then the insertion is
  undone, and point remains on the "g", as expected.

doesn't work as expected in Emacs 25.  This is a regression I'd like
to fix before 25.1 is released.

And this part of 1095/21722 _is_ solved by Phillip's patch, so I think
we should install it on the release branch.  However, I'd like Stefan
to look over the patch before we install it.