call baton.Sync in Reposurgeion.PostCmd, because the prompt is not printed via the baton

Sometimes the prompt appears to be intermingled with output from some command, particularly if that output has many lines. This is because Kommandant prints the prompt to stdout rather than to to the baton. (Also, the baton would need more work in order to do the prompt itself.) Calling the Sync method ensures that Kommandant waits for the baton to catch up.