diff -r f6eff9e4de80 -r 688fc33e105d mercurial/configitems.py
--- a/mercurial/configitems.py	Wed Feb 27 10:49:25 2019 +0100
+++ b/mercurial/configitems.py	Wed Feb 27 12:40:18 2019 +0100
@@ -983,6 +983,9 @@
     default=True,
     alias=[('format', 'aggressivemergedeltas')],
 )
+coreconfigitem('storage', 'revlog.reuse-external-delta',
+    default=True,
+)
 coreconfigitem('storage', 'revlog.reuse-external-delta-parent',
     default=None,
 )