title: Git hash duplicates
body: <p>Git allows to retrieve the hash of the commit with commands like:</p>
  
  <pre><code>git rev-parse HEAD
  </code></pre>
  
  <p>which gives <code>33b316c</code>
  or </p>
  
  <pre><code>git rev-parse --short HEAD
  </code></pre>
  
  <p>which gives <code>33b316cbeeab3d69e79b9fb659414af4e7829a32</code>
  I know that long hashes in practice will never collide. </p>
  
  <p>In practice, the short hashes are used much more often. I'd like to know what's the probability for the short ones to collide? Does git take any measures to overcome possible collisions (when for example using <code>git checkout</code>)?</p>