xgettext: Wrap location comments to 79 characters

xgettext: Wrap location comments to 79 characters

Previously, messages wre wrapped to 79 characters, but location
comments were wrapped to 78 characters.
* gettext-tools/src/write-po.c (message_print_comment_filepos): Fix
off-by-one in calculating line-wrapping width of location comment.

* gettext-tools/tests/xgettext-13: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add new test.
* gettext-tools/tests/mm-viet.out: Adjust to the recent line-wrapping
behavior change.