store: refactor the fncache handling
put all the fncache file writing and reading in
the same place.
{header}
<!-- Changelog -->
<id>{urlbase}{url}</id>
<link rel="self" href="{urlbase}{url}atom-log"/>
<link rel="alternate" href="{urlbase}{url}"/>
<title>{repo|escape} Changelog</title>
{latestentry%feedupdated}
{entries%changelogentry}
</feed>