diff -r 592a3cc1ebc4 -r 0314e02efa25 mercurial/configitems.py
--- a/mercurial/configitems.py	Sun Oct 08 20:16:09 2017 +0200
+++ b/mercurial/configitems.py	Sun Oct 08 20:42:19 2017 +0200
@@ -194,6 +194,9 @@
 coreconfigitem('experimental', 'editortmpinhg',
     default=False,
 )
+coreconfigitem('experimental', 'maxdeltachainspan',
+    default=-1,
+)
 coreconfigitem('experimental', 'nonnormalparanoidcheck',
     default=False,
 )