Check for parent/mod as well as parent.mod in findext. Debian/Ubuntu ship a system-wide hgrc that loads extensions using the syntax hgext/extension=, rather than hgext.extension=. Although the extensions work correctly, hg help <extension> doesn't understand this form without this patch.

Content-type: text/xml

<rss version="2.0">
  <channel>
    <link>#url#</link>
    <language>en-us</language>