backout: allow backout of merge changeset with --parent option.
--parent allows to choose which parent of merge to revert to.
<item>
<title>#tag|escape#</title>
<link>#url#?cs=#node|short#</link>
<description><![CDATA[#tag|strip|escape|addbreaks#]]></description>
<pubDate>#date|rfc822date#</pubDate>
</item>