simplify tracing mechanics by simply going through an indirection

--HG--
branch : more_plugin

_pytest/helpconfig.py

@@ -34,13 +34,15 @@ def pytest_cmdline_parse():
             pytest.__version__, py.__version__,
             ".".join(map(str, sys.version_info)),
             os.getcwd(), config._origargs))
-        config.pluginmanager.set_tracing(debugfile.write)
+        config.trace.root.setwriter(debugfile.write)
+        undo_tracing = config.pluginmanager.enable_tracing()
         sys.stderr.write("writing pytestdebug information to %s\n" % path)
         def unset_tracing():
             debugfile.close()
             sys.stderr.write("wrote pytestdebug information to %s\n" %
                              debugfile.name)
             config.trace.root.setwriter(None)
+            undo_tracing()
         config.add_cleanup(unset_tracing)
 
 def pytest_cmdline_main(config):