Fix performance regression in addremove command. When I rewrote addremove, I lazily put a call to repo.changes in, which was unnecessary and slow. This is a new rewrite, preserving the file name behaviour, but replacing the call to repo.changes with a walk, which is much cheaper, and avoids calls to os.stat on all but files that have probably been deleted.

#header#
    <title>#repo|escape#: #file# history</title>
    <description>#file# revision history</description>
    #entries#
  </channel>
</rss>