dispatch: turn off custom debugger for HGPLAIN mode

Some debuggers, such as ipdb, load escape codes and color codes even when later
turned off. This will affect scripts that do simple parsing and can't handle
escape codes. Therefore, we only load a custom debugger if ui.plain() is false.

mercurial/dispatch.py

@@ -106,8 +106,9 b' def _runcatch(req):'
                 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: