util: Filter out message context of untranslated strings

Commit 5a47ad837b changed the definition of NC_() to include the
message context, so that gettext can correctly disambiguate strings
like "S". However this does only work when gettext actually returns
a translation from the catalogue; if the string is not found, we
need to strip the context again to not have it show up in the UI.

https://bugzilla.gnome.org/show_bug.cgi?id=745521

src/shell-util.c

@@ -223,11 +223,13 @@ shell_util_translate_time_string (const char *str)
 {
   const char *locale = g_getenv ("LC_TIME");
   const char *res;
+  char *sep;
 
   if (locale)
     setlocale (LC_MESSAGES, locale);
 
-  res = gettext (str);
+  sep = strchr (str, '\004');
+  res = g_dpgettext (NULL, str, sep ? str - sep : 0);
 
   setlocale (LC_MESSAGES, "");