diff -r cab34bda259e -r 346781c41597 mercurial/configitems.py
--- a/mercurial/configitems.py	Fri Oct 20 14:15:46 2017 -0700
+++ b/mercurial/configitems.py	Sat Oct 21 13:04:58 2017 +0900
@@ -848,6 +848,9 @@
 coreconfigitem('ui', 'debugger',
     default=None,
 )
+coreconfigitem('ui', 'editor',
+    default=dynamicdefault,
+)
 coreconfigitem('ui', 'fallbackencoding',
     default=None,
 )