title: Auto pull git repositories for opengrok indexing
body: <p>I've installed opengrok from a docker image on an Ubuntu server.
  Inside /var/opengrok/src, I cloned the desired repositories.
  I'd like to auto pull and auto index changes periodically.
  I'm not at ease with cron, is there a way to tell opengrok to pull before indexing?</p>
  <ul>
  <li>/var/opengrok/
  <ul>
  <li>docker-compose.yml</li>
  <li>etc/</li>
  <li>data/</li>
  <li>src/
  <ul>
  <li>repo1-master/   &lt;- need to <code>git pull</code></li>
  <li>repo1-dev/      &lt;- need to <code>git pull</code></li>
  <li>repo2-master/   &lt;- need to <code>git pull</code></li>
  <li>repo2-dev/      &lt;- need to <code>git pull</code></li>
  </ul>
  </li>
  </ul>
  </li>
  </ul>