--- a/mercurial/configitems.py	Sat Mar 03 12:39:14 2018 -0800
+++ b/mercurial/configitems.py	Sat Mar 03 12:39:14 2018 -0800
@@ -577,6 +577,9 @@
 coreconfigitem('experimental', 'sshpeer.advertise-v2',
     default=False,
 )
+coreconfigitem('experimental', 'xdiff',
+    default=False,
+)
 coreconfigitem('extensions', '.*',
     default=None,
     generic=True,