mercurial/ui.py

--- a/mercurial/ui.py	Mon Jul 16 00:32:33 2018 -0400
+++ b/mercurial/ui.py	Mon Jul 16 11:38:56 2018 -0700
@@ -1214,7 +1214,7 @@
                 "Feature %s does not handle all default interfaces" %
                 feature)
 
-        if self.plain():
+        if self.plain() or encoding.environ.get('TERM') == 'dumb':
             return "text"
 
         # Default interface for all the features