title: git update-index --assume-unchanged not working for me
body: <p>I am in a project building a web app. We use Git for version control and Gulp for task automation. I would like to temporarily ignore files to a certain folder, the build folder. We tend to commit changes to that folder at the end of the day (or at the end of a coding session).</p>
  
  <p>After some research I found the following command to start ignoring changes to a file:<br>
  <code>$ git update-index --assume-unchanged path/to/file</code></p>
  
  <p>When I run that code to ignore the build folder I see on the terminal: <code>Ignoring path build/</code>  </p>
  
  <p>But it still shows changes not staged for that folder so that I have to run <code>git checkout -- build/</code> before each commit.</p>
  
  <p>What am I doing wrong?</p>
  
  <p>I am using git 2.9.3 on a PC with elementary OS (Ubuntu-based distro).</p>