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

[Stefan Kangas]

Eli Zaretskii <eliz@gnu.org> writes:

>> The attached patch makes 'sort-lines' sort the entire buffer when there
>> is no region.
>
> How frequently did you see a buffer in Emacs with no region in it?
> IME, it takes about 2 commands since buffer creation to have a region
> in it.  So I predict users to be tripped by this feature quite a lot:
> they will think there's no region, invoke the command, and get some
> arbitrary region sorted.  Requiring them to always provide the region
> avoids this pitfall, since the user is forced to make sure the region
> is where he or she wants it.  And marking the whole buffer is just 2
> key-presses away.
>
> So I think this change will annoy more than help.

Sorry, the correct terminology I should have used is "active region".
This is as tested by `use-region-p'.  Compare the test I introduce also
to the one in `flush-lines', where I lifted it from.

Best regards,
Stefan Kangas