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

[Drew Adams <drew.adams@oracle.com>]

> > > So if the user has transient-mark-mode disabled (which is what I do),
> > > the command will always sort the entire buffer?  Or did I
> > > misunderstand something?
> >
> > I didn't consider that case, but you are correct.  The behavior would be
> > like that documented in `flush-lines':
> >
> >     Interactively, in Transient Mark mode when the mark is active, operate
> >     on the contents of the region.  Otherwise, operate from point to the
> >     end of (the accessible portion of) the buffer.
> >
> > I'm not sure the `flush-lines' behavior makes sense, though.  It looks
> > like you can't run it on a region without transient-mark-mode.  So maybe
> > we should fix that as well.
> 
> I think we should indeed fix flush-lines as well, yes.
> 
> > (Note that `flush-lines' operates from point to point-max, but
> > `sort-lines' is proposed here to operate on the entire buffer instead as
> > per previous discussion in this bug.)
> 
> I guess due to historical reasons?

Apologies for not following this thread and perhaps
misunderstanding at this point.

If `transient-mark-mode' is off then there is no notion
of an "active region".  There is just the region.

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.

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