title: gitignore doesn't ignore files in repository
body: <p>I have defined the following <code>.gitignore</code>:</p>
  
  <pre><code>/aclocal.m4
  /config.guess
  /config.sub
  </code></pre>
  
  <p>However after reconfiguration and rebuild, <code>git status</code> still reports:</p>
  
  <pre><code># On branch master
  # Changed but not updated:
  #   (use "git add/rm &lt;file&gt;..." to update what will be committed)
  #   (use "git checkout -- &lt;file&gt;..." to discard changes in working directory)
  #
  #       modified:   aclocal.m4
  #       modified:   config.guess
  #       modified:   config.sub
  ...
  </code></pre>
  
  <p>I'm expecting it to say something like:</p>
  
  <pre><code>#       deleted:    aclocal.m4
  ...
  </code></pre>
  
  <p>What am I doing wrong here?</p>