hgweb: correct an earlier error of mine - `start` should be bytes

Gets hgweb very close to working with Python 3.

Differential Revision: https://phab.mercurial-scm.org/D1134

mercurial/hgweb/hgweb_mod.py

@@ -187,7 +187,7 b' class requestcontext(object):'
         if style == styles[0]:
             vars['style'] = style
 
-        start = r'&' if req.url[-1] == r'?' else r'?'
+        start = '&' if req.url[-1] == r'?' else '?'
         sessionvars = webutil.sessionvars(vars, start)
 
         if not self.reponame: