msginit: Fix encoding of second title line, part 2.

msginit: Fix encoding of second title line, part 2.

Reported by Matilin Torre <watilin@protonmail.com> in
<https://lists.gnu.org/archive/html/bug-gettext/2025-07/msg00024.html>.

To reproduce the problem:
$ LC_ALL=de_DE.ISO-8859-1 msginit -l fr_FR.ISO-8859-1 -i some_utf-8.pot -o fr.po

* gettext-tools/src/msginit.c (output_charset): New variable.
(content_type): Use output_charset.
(get_title): Use output_charset instead of ignoring was_utf8.
(fill_header): Initialize output_charset. Moved logic from content_type()
to here.