/usr/share/mercurial/templates/rss/filelogentry.tmpl is in mercurial-common 3.7.3-1ubuntu1.
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>{desc|strip|firstline|strip|escape}</title>
<link>{urlbase}{url|urlescape}log{node|short}/{file|urlescape}</link>
<description><![CDATA[{desc|strip|escape|websub|addbreaks|nonempty}]]></description>
<author>{author|obfuscate}</author>
<pubDate>{date|rfc822date}</pubDate>
</item>
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