arc feature set up git tracking branch

Summary:
When using arc feature, it should set up the tracking branch to be
master.

Test Plan:
  ./bin/arc feature tracking

Reviewers: vrana, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5169

src/workflow/ArcanistFeatureWorkflow.php

@@ -152,7 +152,7 @@ EOTEXT
         }
       } else {
         $exec = $api->execManualLocal(
-          'checkout -b %Ls',
+          'checkout --track -b %Ls',
           $names);
       }