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.

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<item>

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    <title>#tag|escape#</title>

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    <link>{urlbase}{url}rev/{node|short}</link>

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    <description><![CDATA[#tag|strip|escape|addbreaks#]]></description>

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    <pubDate>#date|rfc822date#</pubDate>

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</item>