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