--- a/mercurial/configitems.py	Fri Oct 05 20:24:07 2018 +0300
+++ b/mercurial/configitems.py	Fri Oct 05 22:19:19 2018 +0300
@@ -538,6 +538,9 @@
 coreconfigitem('experimental', 'mmapindexthreshold',
     default=None,
 )
+coreconfigitem('experimental', 'narrow',
+    default=False,
+)
 coreconfigitem('experimental', 'nonnormalparanoidcheck',
     default=False,
 )