/usr/share/mercurial/templates/raw/changeset.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.
1 2 3 4 5 6 7 8 9 | {header}
# HG changeset patch
# User {author}
# Date {date|hgdate}
# Node ID {node}
{parent%changesetparent}
{desc}
{diff}
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