title: Named stages as git checkout-index parameters
body: <pre><code>git checkout-index --help 
  </code></pre>
  
  <p>says</p>
  
  <blockquote>
    <p>--stage=|all 
    Instead of checking out unmerged entries, copy out the files from named stage.  must be between 1 and 3. Note: --stage=all automatically implies --temp. </p>
  </blockquote>
  
  <p>I would like to understand what are the "named stages" corresponding to numbers from 1 to 3?</p>