patch 8.2.1548: cannot move position of "%%" in message translations

Problem:    Cannot move position of "%%" in message translations. (Emir Sarı)
Solution:   Improve the check script.
diff --git a/src/po/check.vim b/src/po/check.vim
index 454a053..f94070f 100644
--- a/src/po/check.vim
+++ b/src/po/check.vim
@@ -22,6 +22,7 @@
 
   " remove '%', not used for formatting.
   let idline = substitute(idline, "'%'", '', 'g')
+  let idline = substitute(idline, "%%", '', 'g')
 
   " remove '%' used for plural forms.
   let idline = substitute(idline, '\\nPlural-Forms: .\+;\\n', '', '')