title: Using `git submodule update`
body: <p>Let's say I have <code>project_a</code> (git repo), which contains files &amp; another <code>project_b</code> (git repo).</p>
  
  <pre><code>project_a
   --&gt; files
   --&gt; project_b
  </code></pre>
  
  <p>As the <a href="http://git-scm.com/docs/git-submodule" rel="nofollow">git-scm</a> book demonstrates, I can run:</p>
  
  <pre><code>cd project_a
  git submodule init
  git submodule update // clones project_b repo specified in '.gitsubmodules'
  </code></pre>
  
  <p>to populate the initially empty <code>project_b</code> directory within <code>project_a</code>.</p>
  
  <p>Rather than run <code>git submodule update</code> to get the latest from <code>project_b</code>, I'd prefer to clone the latest version of <code>project_b</code> at a point in time, and then have <code>project_a</code> only use that version (unless, of course, I do a <code>git pull</code> in <code>project_b</code>).</p>
  
  <p>How can I do this?</p>