mercurial/configitems.py

--- a/mercurial/configitems.py	Fri Jan 18 13:13:30 2019 -0800
+++ b/mercurial/configitems.py	Tue Feb 19 15:42:45 2019 -0800
@@ -482,6 +482,9 @@
 coreconfigitem('experimental', 'copytrace.sourcecommitlimit',
     default=100,
 )
+coreconfigitem('experimental', 'copies.read-from',
+    default="filelog-only",
+)
 coreconfigitem('experimental', 'crecordtest',
     default=None,
 )