title: Add a file to a git repository but ignore future changes on it
body: <p>I want to add an <code>overrides.json</code>.  That allows developers to make changes to a config object locally.  I want it added to the repository in a default state but I want future changes on it to be ignored.</p>
  
  <p>It seems like <code>.gitignore</code> forces me to remove it from the branch before I can ignore it.  I know about <code>git update-index</code> but each developer would have to do this manually.  To clarify, I do not want the file removed; I actually want the file to be added to repository but untracked after that.</p>
  
  <p>I can't simply remove it because <code>GET</code> requests to a missing file throws ugly error messages in the browser's javascript console.</p>