configitems: register the 'web.labels' config

mercurial/configitems.py

@@ -742,6 +742,9 b" coreconfigitem('web', 'guessmime',"
 coreconfigitem('web', 'hidden',
     default=None,
 )
+coreconfigitem('web', 'labels',
+    default=list,
+)
 coreconfigitem('web', 'accesslog',
     default='-',
 )