title: How to sync a git repo to a mirrored repo
body: <p>I want to mirror a git repo to my local repo, and wish to 
  timely sync them, I did like this:</p>
  
  <pre><code>git clone --mirror source_url source_repo
  cd source_repo
  git push --mirror local_url
  </code></pre>
  
  <p>Now I have mirrored the source repo to my local repo, I could clone my
  local repo like this:</p>
  
  <pre><code>git clone local_url local_repo
  </code></pre>
  
  <p>But since the source repo is going update, how could I keep them sync?</p>