hgweb: use registrar to declare "websub" template filter

Prepares for adding 'intype' attribute.

mercurial/hgweb/hgweb_mod.py

@@ -27,6 +27,7 b' from .. import ('
     hook,
     profiling,
     pycompat,
+    registrar,
     repoview,
     templatefilters,
     templater,
@@ -170,6 +171,9 b' class requestcontext(object):'
                              or req.apppath
                              or self.repo.root)
 
+        filters = {}
+        templatefilter = registrar.templatefilter(filters)
+        @templatefilter('websub')
         def websubfilter(text):
             return templatefilters.websub(text, self.websubtable)
 
@@ -191,7 +195,7 b' class requestcontext(object):'
         }
         tres = formatter.templateresources(self.repo.ui, self.repo)
         tmpl = templater.templater.frommapfile(mapfile,
-                                               filters={'websub': websubfilter},
+                                               filters=filters,
                                                defaults=defaults,
                                                resources=tres)
         return tmpl