title: ! [remote rejected] errors after mirroring a git repository
body: <p>I'm following this documentation:
  <a href="https://help.github.com/articles/duplicating-a-repository/">https://help.github.com/articles/duplicating-a-repository/</a></p>
  
  <pre><code>git clone --mirror https://github.com/exampleuser/repository-to-mirror.git
  
  cd repository-to-mirror.git
  
  git push --mirror https://github.com/exampleuser/mirrored
  </code></pre>
  
  <p>The output shows that the repository is pushed as a mirror, but for some reason I'm getting these errors as well:</p>
  
  <pre><code> ! [remote rejected] refs/pull/1/head -&gt; refs/pull/1/head (deny updating a hidden ref)
   ! [remote rejected] refs/pull/1/merge -&gt; refs/pull/1/merge (deny updating a hidden ref)
  </code></pre>
  
  <p>What are these errors? Can I assume the repository was mirrored ?</p>