po-mode: Fix syntax highlighting of comment lines with double-quotes.
Reported by Vincent Lefèvre <vincent@vinc17.net> in
<https://lists.gnu.org/archive/html/bug-gettext/2020-06/msg00000.html>.
* gettext-tools/emacs/po-mode.el (po-mode-version-string): Bump.
(po-font-lock-keywords): Move the rules that recognize comments to the
beginning.
