bug#13521: `sort-lines' on the empty region

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 5 Sep 2020 07:36:55 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> Cc: xfq.free@gmail.com, rgm@gnu.org, monnier@iro.umontreal.ca,
>         13521@debbugs.gnu.org
> 
> I doubt that someone who has `transient-mark-mode' off
> would ever want commands such as `flush-lines' and
> `sort-lines' to act on the region.  And if they did, I
> expect they'd just narrow to it.

Please don't doubt, and please don't impose unnecessary commands on
users who have transient-mark-mode off.

> Anything that works on "the active region" is something
> that makes sense only when `transient-mark-mode' is on
> (IMHO).

The important point here is that sort-lines worked on the region, even
if inactive, before the proposed changes, so restricting it now only
to active regions would be a backward-incompatible change of behavior.