title: Vim loses undo history when changing buffers
body: <p>If I'm working in a file, change to another buffer, and then change back, I have lost my undo history.</p>
  
  <ol>
  <li><code>vim File1.txt</code> - make a bunch of changes &amp; save.</li>
  <li>Open new buffer - <code>:e test.txt</code></li>
  <li>Switch back to File1.txt - <code>:b#</code></li>
  <li>Undo history is gone.</li>
  </ol>
  
  <p>Any workarounds for this?</p>