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

mercurial/hgweb/hgwebdir_mod.py

@@ -510,7 +510,7 b' class hgwebdir(object):'
         if style == styles[0]:
             vars['style'] = style
 
-        sessionvars = webutil.sessionvars(vars, r'?')
+        sessionvars = webutil.sessionvars(vars, '?')
         logourl = config('web', 'logourl')
         logoimg = config('web', 'logoimg')
         staticurl = (config('web', 'staticurl')