configitems: register the 'acl.config' config

hgext/acl.py

@@ -199,6 +199,7 b' from mercurial.i18n import _'
 from mercurial import (
     error,
     match,
+    registrar,
     util,
 )
 
@@ -210,6 +211,14 b' urlreq = util.urlreq'
 # leave the attribute unspecified.
 testedwith = 'ships-with-hg-core'
 
+configtable = {}
+configitem = registrar.configitem(configtable)
+
+# deprecated config: acl.config
+configitem('acl', 'config',
+    default=None,
+)
+
 def _getusers(ui, group):
 
     # First, try to use group definition from section [acl.groups]