--- a/mercurial/configitems.py	Wed Jan 17 16:52:13 2018 +0100
+++ b/mercurial/configitems.py	Wed Jan 17 17:07:55 2018 +0100
@@ -568,6 +568,9 @@
 coreconfigitem('experimental', 'treemanifest',
     default=False,
 )
+coreconfigitem('experimental', 'update.atomic-file',
+    default=False,
+)
 coreconfigitem('extensions', '.*',
     default=None,
     generic=True,