branchcache: move the filename to a class attribute
This prepare the introduction of more variant of cache.
<item>
<title>{desc|strip|firstline|strip|escape}</title>
<link>{urlbase}{url|urlescape}log/{node|short}/{file|urlescape}</link>
<description><![CDATA[{desc|strip|escape|websub|addbreaks|nonempty}]]></description>
<author>{author|obfuscate}</author>
<pubDate>{date|rfc822date}</pubDate>
</item>