Add revlog.parentrevs function. This allows one to walk the revision graph using only revision numbers, which can be faster than using revision hashes, especially for RevlogNG, where the parents of a revision are stored as revision numbers.

<item>
    <title>#tag|escape#</title>
    <link>#url#?cs=#node|short#</link>
    <description><![CDATA[#tag|strip|escape|addbreaks#]]></description>
    <pubDate>#date|rfc822date#</pubDate>
</item>