/usr/share/mercurial/templates/atom/header.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.
1 2 | <?xml version="1.0" encoding="{encoding}"?>
<feed xmlns="http://www.w3.org/2005/Atom">
|