This r'' is leaking into the templating layer and causing an
assertion failure.
The r'' was added in d1fccbd50fcd (October 2017). Similar code
in hgweb_mod.py was also changed in that changeset. hgweb_mod.py was
updated in ec46415ed826 (March 2018) to use webutil.sessionvars(),
which doesn't use raw strings.