title: Prevent git submodule update from deleting submodule committed changes
body: <p>This seems a to be a common issue, but i just can't find a reasonable solution:<br>
  IMHO, the worse problem with <a href="http://schacon.github.com/git/user-manual.html#submodules" rel="nofollow">git submodules</a> is where submodule committed but not pushed changes are easily deleted with the containing repository usage of <code>git submodule update</code>. Quoting:</p>
  
  <blockquote>
    <p>It’s not safe to run git submodule update if you’ve made and committed
    changes within a submodule without checking out a branch first. They
    will be silently overwritten:      </p>
  </blockquote>
  
  <p>(...followed by an example there).</p>
  
  <p>So, to the question-<br>
  Let <code>super</code> be a repository root folder and <code>submod</code> be a child folder containing a submodule:    </p>
  
  <pre><code>-super   
       -submod
  </code></pre>
  
  <p>When i commit in the submodule but not push, and then run <code>git submodule update</code> i get my <em>committed</em> changes in submod <strong>deleted</strong> (oh, the horror).<br>
  How can i add a confirmation <code>are you sure</code> question when i'm doing this?
  hooks, scripts or any other applicable ideas are more the welcome.</p>
  
  <p>TIA</p>