mercurial/dispatch.py

--- a/mercurial/dispatch.py	Sun Nov 17 12:18:26 2013 +0100
+++ b/mercurial/dispatch.py	Mon Nov 25 12:20:39 2013 -0500
@@ -106,8 +106,9 @@
                 for cfg in cfgs:
                     req.repo.ui.setconfig(*cfg)
 
+            # if we are in HGPLAIN mode, then disable custom debugging
             debugger = ui.config("ui", "debugger")
-            if not debugger:
+            if not debugger or ui.plain():
                 debugger = 'pdb'
 
             try: