configitems: register the 'web.hidden' config

mercurial/configitems.py

@@ -739,6 +739,9 b" coreconfigitem('web', 'deny_push',"
 coreconfigitem('web', 'guessmime',
     default=False,
 )
+coreconfigitem('web', 'hidden',
+    default=None,
+)
 coreconfigitem('web', 'accesslog',
     default='-',
 )