diff -r 169c386cc9d8 -r f6eff9e4de80 mercurial/configitems.py
--- a/mercurial/configitems.py	Sat Mar 02 09:41:17 2019 +0900
+++ b/mercurial/configitems.py	Wed Feb 27 10:49:25 2019 +0100
@@ -983,6 +983,9 @@
     default=True,
     alias=[('format', 'aggressivemergedeltas')],
 )
+coreconfigitem('storage', 'revlog.reuse-external-delta-parent',
+    default=None,
+)
 coreconfigitem('server', 'bookmarks-pushkey-compat',
     default=True,
 )