revlog: move comment about commonancestorsheads where it's used
Differential Revision: https://phab.mercurial-scm.org/D3928
<entry>
<title>{tag|escape}</title>
<link rel="alternate" href="{urlbase}{url|urlescape}rev/{node|short}"/>
<id>{urlbase}{url|urlescape}#tag-{node}</id>
<updated>{date|rfc3339date}</updated>
<published>{date|rfc3339date}</published>
<content type="text">{tag|strip|escape}</content>
</entry>