configitems: register the 'experimental.allowdivergence' config

mercurial/configitems.py

@@ -155,6 +155,9 b" coreconfigitem('email', 'from',"
 coreconfigitem('email', 'method',
     default='smtp',
 )
+coreconfigitem('experimental', 'allowdivergence',
+    default=False,
+)
 coreconfigitem('experimental', 'bundle-phases',
     default=False,
 )