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

[Stefan Kangas <stefan@marxist.se>]

Eli Zaretskii <eliz@gnu.org> writes:

> 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.

(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.)