Remove empty undo group

removeEmptyUndoGroup.m

   id undostack = object_getIvar(undoManager, class_getInstanceVariable([NSUndoManager class], NEVER_TRANSLATE("_undoStack")));
   id headAction = object_getIvar(undostack, class_getInstanceVariable([undostack class], NEVER_TRANSLATE("_head")));
   id nextAction = object_getIvar(headAction, class_getInstanceVariable([headAction class], NEVER_TRANSLATE("next")));

   const char *beginGroupName = NEVER_TRANSLATE("_NSUndoBeginMark");
   const char *nextActionName = object_getClassName(nextAction);
   if( strcmp(beginGroupName,nextActionName) == 0 )
   {
      [undoManager undo];
   }