dispatch: disable demandimport when invoking the debugger

Turns out that demandimport confuses pudb, which does some pretty
complicated imports. I think it's reasonable to disable demandimport
here.

mercurial/dispatch.py

@@ -181,8 +181,8 b' def _runcatch(req):'
                     debugtrace[debugger] == debugtrace['pdb']):
                     ui.warn(_("%s debugger specified "
                               "but its module was not found\n") % debugger)
-
-                debugtrace[debugger]()
+                with demandimport.disabled():
+                    debugtrace[debugger]()
             try:
                 return _dispatch(req)
             finally: