diff -r 51f10e6d66c7 -r e2697acd9381 mercurial/cmdutil.py
--- a/mercurial/cmdutil.py	Tue Oct 02 12:43:54 2018 -0700
+++ b/mercurial/cmdutil.py	Wed Oct 03 10:27:44 2018 -0700
@@ -861,7 +861,7 @@
     if isinstance(ctxorbool, bool):
         if ctxorbool:
             return baseformname + ".merge"
-    elif 1 < len(ctxorbool.parents()):
+    elif len(ctxorbool.parents()) > 1:
         return baseformname + ".merge"
 
     return baseformname + ".normal"