branch heads: fix regression introduced in e67e5b60e55f (issue1726)
For merge nodes it is not adequate to only check a single possible
branch head for whether it is an ancestor of the latest head, but it
needs to be done for each possible branch head.
{header}
<id>{urlbase}{url}atom-log/tip/{file|escape}</id>
<link rel="self" href="{urlbase}{url}atom-log/tip/{file|urlescape}"/>
<title>{repo|escape}: {file|escape} history</title>
{latestentry%feedupdated}
{entries%changelogentry}
</feed>