configitems: register the 'notify.mbox' config

hgext/notify.py

@@ -176,6 +176,9 b" configitem('notify', 'maxdiff',"
 configitem('notify', 'maxsubject',
     default=67,
 )
+configitem('notify', 'mbox',
+    default=None,
+)
 
 # template for single changeset can include email headers.
 single_template = '''