title: Using Vim's persistent undo?
body: <p>One of the new features in Vim 7.3 is 'persistent undo', which allows for the undotree to be saved to a file when exiting a buffer.</p>
  
  <p>Unfortunately, I haven't quite been able to get it properly enabled, or I must be using it wrong.  Here's what I've tried so far:</p>
  
  <p>I added the following to ~/.vimrc</p>
  
  <pre><code>set undofile                " Save undos after file closes
  set undodir=$HOME/.vim/undo " where to save undo histories
  set undolevels=1000         " How many undos
  set undoreload=10000        " number of lines to save for undo
  </code></pre>
  
  <p>After this, I supposedly should be able to open any file, edit it, then save-close it, and when I open it again I should be able to undo/redo as if I'd never left.  Unfortunately, this doesn't seem to be the case, as no undofile is ever written. </p>
  
  <p>Notes:</p>
  
  <ol>
  <li><p>I'm on Win 7 using Vim 7.3 from the Vim without cream project. Persistent undo is baked-in.</p></li>
  <li><p>$HOME/.vim/undo exists on my file system</p></li>
  </ol>