diff -r cd2342302928 -r 9037c29e9f53 mercurial/configitems.py
--- a/mercurial/configitems.py	Fri Feb 02 23:20:55 2018 -0500
+++ b/mercurial/configitems.py	Wed Jan 17 17:35:05 2018 -0800
@@ -743,6 +743,16 @@
     generic=True,
     priority=-1,
 )
+coreconfigitem('merge-tools', br'.*\.mergemarkers$',
+    default='basic',
+    generic=True,
+    priority=-1,
+)
+coreconfigitem('merge-tools', br'.*\.mergemarkertemplate$',
+    default=dynamicdefault,  # take from ui.mergemarkertemplate
+    generic=True,
+    priority=-1,
+)
 coreconfigitem('merge-tools', br'.*\.priority$',
     default=0,
     generic=True,