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.
<entry>
<title>{tag|escape}</title>
<link rel="alternate" href="{urlbase}{url}rev/{node}"/>
<id>{urlbase}{url}#tag-{node}</id>
<updated>{date|rfc3339date}</updated>
<published>{date|rfc3339date}</published>
<content type="text">{tag|strip|escape}</content>
</entry>