convert: make contents of "extra" dict available from sources, for sinks. This breaks hash preservation for hg->hg conversion, as each converted change gets a convert_revision item added to its extra dict. Ugh.

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