helm-gtags-update-tags Generate new TAG file in selected directory with C-uC-u

 ```
(defun helm-gtags-update-tags ()
  "Update TAG file. Update All files with `C-u' prefix,
Generate new TAG file in selected directory with `C-uC-u'"

```
  "in `after-save-hook' current-time - last-time must bigger than 1min(default)
then `helm-gtags-update-tags' will be called,nil means update immidiately"