title: Git warn before committing if string appears in source (or diff)
body: <p>I would like to be prevented† when staging‡ in a <code>git</code> repo if the changes I am about to commit contain a certain string (say, <code>@todo</code> or <code>@hack</code>).</p>
  
  <p>Can someone show me how to achieve this?</p>
  
  <p><em>† or warned.</em><br>
  <em>‡ or when committing.</em></p>