hgweb: fix handling of path for old style template
- path from old style are prefixed by '/', make cleanpath strip them
- make manifest() use relative paths, that was the only function using
'/' prefixed paths
<item>
<title>#tag|escape#</title>
<link>{urlbase}{url}rev/{node|short}</link>
<description><![CDATA[#tag|strip|escape|addbreaks#]]></description>
<pubDate>#date|rfc822date#</pubDate>
</item>