msgmerge: Make --sort-by-file sort messages without file reference correctly.
Reported by <wangqr@wangqr.tk> in <https://savannah.gnu.org/bugs/?58778>.
* gettext-tools/src/write-catalog.c (cmp_by_filepos): When a->filepos_count and
b->filepos_count are both 0, compare the msgid strings.
* gettext-tools/tests/msgmerge-29: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.
