mercurial/templates/rss/error.tmpl

discovery: prevent crash caused by prune marker having no parent data

If a marker has no parent information, parents field is set to None, which
caused TypeError. I think this shouldn't normally happen, but somehow
I have such marker in my repo.

{header}
    <title>Error</title>
    <description>Error</description>
    <item>
      <title>Error</title>
      <description>{error|escape}</description>
      <guid>https://mercurial-scm.org/#error</guid>
    </item>
  </channel>
</rss>