title: git checkout with commit ID?
body: <p>I'm a git newbie, I've cloned the repository and I've been given a git commit ID.  When I try to do:</p>
  <pre><code>$ git checkout 2ae0568
  </code></pre>
  <p>I get:</p>
  <pre><code>error: pathspec '2ae0568' did not match any file(s) known to git 
  </code></pre>
  <p>What haven't I done?</p>