title: How to read git log graph
body: <p>In the git community book, it says</p>
  
  <blockquote>
    <p>Another interesting thing you can do is visualize the commit graph with the '--graph' option, like so:</p>
  
  <pre><code>$ git log --pretty=format:'%h : %s' --graph
  * 2d3acf9 : ignore errors from SIGCHLD on trap
  *   5e3ee11 : Merge branch 'master' of git://github.com/dustin/grit
  |\
  | * 420eac9 : Added a method for getting the current branch.
  * | 30e367c : timeout code and tests
  * | 5a09431 : add timeout protection to grit
  * | e1193f8 : support for heads with slashes in them
  |/
  * d6016bc : require time for xmlschema
  </code></pre>
    
    <p>It will give a pretty nice ASCII representation of the commit history lines.</p>
  </blockquote>
  
  <p>How should I read this graph? How does <code>420eac9</code> differ from the rest?</p>