/usr/share/mercurial/templates/gitweb/ is in mercurial-common 4.0-1+deb9u1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/mercurial/templates/gitweb/bookmarks.tmpl
/usr/share/mercurial/templates/gitweb/branches.tmpl
/usr/share/mercurial/templates/gitweb/changelog.tmpl
/usr/share/mercurial/templates/gitweb/changelogentry.tmpl
/usr/share/mercurial/templates/gitweb/changeset.tmpl
/usr/share/mercurial/templates/gitweb/error.tmpl
/usr/share/mercurial/templates/gitweb/fileannotate.tmpl
/usr/share/mercurial/templates/gitweb/filecomparison.tmpl
/usr/share/mercurial/templates/gitweb/filediff.tmpl
/usr/share/mercurial/templates/gitweb/filelog.tmpl
/usr/share/mercurial/templates/gitweb/filerevision.tmpl
/usr/share/mercurial/templates/gitweb/footer.tmpl
/usr/share/mercurial/templates/gitweb/graph.tmpl
/usr/share/mercurial/templates/gitweb/header.tmpl
/usr/share/mercurial/templates/gitweb/help.tmpl
/usr/share/mercurial/templates/gitweb/helptopics.tmpl
/usr/share/mercurial/templates/gitweb/index.tmpl
/usr/share/mercurial/templates/gitweb/manifest.tmpl
/usr/share/mercurial/templates/gitweb/map
/usr/share/mercurial/templates/gitweb/notfound.tmpl
/usr/share/mercurial/templates/gitweb/search.tmpl
/usr/share/mercurial/templates/gitweb/shortlog.tmpl
/usr/share/mercurial/templates/gitweb/summary.tmpl
/usr/share/mercurial/templates/gitweb/tags.tmpl