--- a/mercurial/configitems.py	Fri Jun 30 03:42:27 2017 +0200
+++ b/mercurial/configitems.py	Fri Jun 30 03:42:28 2017 +0200
@@ -125,6 +125,9 @@
 coreconfigitem('format', 'obsstore-version',
     default=None,
 )
+coreconfigitem('format', 'usefncache',
+    default=True,
+)
 coreconfigitem('hostsecurity', 'ciphers',
     default=None,
 )