--- a/mercurial/configitems.py	Fri Jun 30 03:42:24 2017 +0200
+++ b/mercurial/configitems.py	Fri Jun 30 03:42:26 2017 +0200
@@ -119,6 +119,9 @@
 coreconfigitem('format', 'manifestcachesize',
     default=None,
 )
+coreconfigitem('format', 'maxchainlen',
+    default=None,
+)
 coreconfigitem('hostsecurity', 'ciphers',
     default=None,
 )