--- a/mercurial/configitems.py	Mon Jul 09 09:50:23 2018 -0400
+++ b/mercurial/configitems.py	Fri Jun 22 01:42:38 2018 +0200
@@ -629,7 +629,7 @@
     generic=True,
 )
 coreconfigitem('format', 'aggressivemergedeltas',
-    default=False,
+    default=True,
 )
 coreconfigitem('format', 'chunkcachesize',
     default=None,