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.

#header#
    <title>#repo|escape# Changelog</title>
    <description>#repo|escape# Changelog</description>
    #entries%changelogentry#
  </channel>
</rss>