po/make_pot: also update PO files

While meson already provides the aegisub-update-po target
for this exact purpose, it insists on also running meson’s
aegisub-pot target first, leading to missing translation strings.

po/make_pot.sh

@@ -54,3 +54,9 @@ do
     | sed 's/[^=]*=\(.*\)/packages\/win_installer\/fragment_strings.iss|1|"\1"/' \
     | maybe_append
 done
+
+for lang in $(cat LINGUAS) ; do
+  # If using gettext < 0.21, run twice to avoid reversing order of old strings
+  # ref: https://savannah.gnu.org/bugs/?58778
+  msgmerge --update --backup=none --no-fuzzy-matching --sort-by-file "$lang".po aegisub.pot
+done