/usr/share/mercurial/templates/rss/branchentry.tmpl is in mercurial-common 4.5.3-1ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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<title>{branch|escape}</title>
<link>{urlbase}{url|urlescape}rev/{node|short}</link>
<description><![CDATA[{branch|strip|escape|addbreaks}]]></description>
<pubDate>{date|rfc822date}</pubDate>
</item>
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