title: git update-index --no-assume-unchanged doesn't work
body: <p>I set the <code>assume-unchanged</code> bit on a couple of files, and now I want to unset them, but that doesn't work.</p>
  
  <pre><code>&gt; git update-index --no-assume-unchanged Gemfile
  &gt; git ls-files -v | grep Gemfile
  S Gemfile
  S Gemfile.lock
  &gt; git status
  # On branch foo
  nothing to commit (working directory clean)
  &gt; 
  </code></pre>
  
  <p>I have changes in my Gemfile of course. Some of them I don't want to commit, others I do. And if anyone else changes Gemfile, I can't merge because of that bit. It really seems to mess a bit too much stuff up.</p>
  
  <p>So I have two questions:</p>
  
  <p>1: How do I unset the bit?
  2: Is there a better way to ignore one specific local change, while allowing other changes to that file to work normally?</p>