patch 8.0.0830: translating messages is not ideal

Problem:    Translating messages is not ideal.
Solution:   Add a remark about obsolete messages.  Use msgfmt in the check
            script. (Christian Brabandt)
diff --git a/src/po/check.vim b/src/po/check.vim
index 3bcbef3..41df8f8 100644
--- a/src/po/check.vim
+++ b/src/po/check.vim
@@ -143,6 +143,16 @@
   endif
 endwhile
 
+" Check that the file is well formed according to msgfmts understanding
+if executable("msgfmt")
+  let filename = expand("%")
+  let a = system("msgfmt --statistics OLD_PO_FILE_INPUT=yes" . filename)
+  if v:shell_error != 0
+    let error = matchstr(a, filename.':\zs\d\+\ze:')+0
+    for line in split(a, '\n') | echomsg line | endfor
+  endif
+endif
+
 if error == 0
   " If all was OK restore the view.
   call winrestview(wsv)