/usr/share/mercurial/templates/atom/ is in mercurial-common 4.0-1+deb9u1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/mercurial/templates/atom/bookmarkentry.tmpl
/usr/share/mercurial/templates/atom/bookmarks.tmpl
/usr/share/mercurial/templates/atom/branchentry.tmpl
/usr/share/mercurial/templates/atom/branches.tmpl
/usr/share/mercurial/templates/atom/changelog.tmpl
/usr/share/mercurial/templates/atom/changelogentry.tmpl
/usr/share/mercurial/templates/atom/error.tmpl
/usr/share/mercurial/templates/atom/filelog.tmpl
/usr/share/mercurial/templates/atom/header.tmpl
/usr/share/mercurial/templates/atom/map
/usr/share/mercurial/templates/atom/tagentry.tmpl
/usr/share/mercurial/templates/atom/tags.tmpl