hgweb: don't use raw string for session vars 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. Differential Revision: https://phab.mercurial-scm.org/D5651