msgmerge: Fix behaviour of --for-msgfmt on PO files with no translations.
Reported by Don Lawrence <dlawrence@iecok.com>
in <https://lists.freedesktop.org/archives/p11-glue/2019-May/000700.html>
via Daiki Ueno
in <https://lists.gnu.org/archive/html/bug-gettext/2019-05/msg00124.html>.
* gettext-tools/src/msgmerge.c (main): Treat force_po like true if for_msgfmt
is true.
* gettext-tools/tests/msgmerge-26: Add test of PO file with no translations.
