diff -r a4483e380c3e -r 0e41f40b01cc mercurial/configitems.py
--- a/mercurial/configitems.py	Thu Apr 04 13:46:49 2019 +0200
+++ b/mercurial/configitems.py	Wed Dec 27 19:49:36 2017 -0800
@@ -488,6 +488,9 @@
 coreconfigitem('experimental', 'copies.read-from',
     default="filelog-only",
 )
+coreconfigitem('experimental', 'copies.write-to',
+    default='filelog-only',
+)
 coreconfigitem('experimental', 'crecordtest',
     default=None,
 )