hgweb: break templater -> templater circular reference

The problem were some functions passed in the "defaults" argument
during the templater creation which use "self.t" directly.  This
creates the cycle:

 hgweb object
  -> templater object
      -> defaults dict
          -> footer function
              -> hgweb object

Instead of completely avoding the cycle, we break it after using
the templater.

mercurial/hgweb/hgweb_mod.py

@@ -806,6 +806,7 b' class hgweb(object):'
                                                "sessionvars": sessionvars
                                                })
 
+        try:
         if not req.form.has_key('cmd'):
             req.form['cmd'] = [self.t.cache['default']]
 
@@ -819,6 +820,8 b' class hgweb(object):'
                 req.write(self.t("error", error=str(inst)))
         else:
             req.write(self.t("error", error='No such method: ' + cmd))
+        finally:
+            self.t = None
 
     def changectx(self, req):
         if req.form.has_key('node'):