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

[Stefan Monnier]

> 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.

Indeed, so the patch should check something like

    (if (or (null transient-mark-mode) (use-region-p))
        (list (region-beginning) (region-end))
      (list (point-min) (point-max)))

`use-region-p` checks `transient-mark-mode` the other way, because it's
used mostly for commands which previously did *not* operate on the
region, unlike `sort-lines`.


        Stefan