msgmerge: Deprecate option '--sort-output'.
Reported by Ben Siemerink <gnu@mail.qoben.com> in
<https://savannah.gnu.org/bugs/?61249>.
* gettext-tools/src/msgmerge.c (main): Warn when option '--sort-output' is used.
(usage): Mark the option '--sort-output' deprecated.
* gettext-tools/doc/msgmerge.texi: Likewise.
* NEWS: Mention it.
