bug#23794: Emacs 25.0.94: Patch to make sort-lines respect visible lines (fairly urgent)

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Weiner <rsw@gnu.org>
> Date: Sat, 18 Jun 2016 11:47:08 -0400
> 
> sort-lines calls forward-line rather than forward-visible line, so if
> you have emacs outline entries that are collapsed/hidden to single lines
> each and you try to sort them, their bodies and subtrees are sorted
> separately because forward-visible-line is not used.
> 
> This patch fixes this problem and also unifies the calling convention of
> forward-visible-line with that of forward-line (allowing it to take an
> optional argument) leading to a cleaner calling convention.
> 
> Please apply it as soon as you can as Hyperbole uses sort-lines to sort
> its contact manager records and right now this doesn't work.  Although,
> sort-subr could be called directly for this application, sort-lines
> should work properly with both visible and invisible text and the patch
> is quite simple.

I don't think we can make such a backward-incompatible change without
(a) an entry in NEWS, (b) suitable changes in the manual(s), and
(c) some way of getting back the old behavior (which could be by way
of having this new behavior as an optional one).

Thanks.