Remove extra lines before global parameters when calling `wp post --help`

See https://github.com/wp-cli/wp-cli/pull/1515#discussion_r21326157


I'm not sure if this is the best way to check when we should add the extra lines.

The first condition works for most of the cases, except for subcommands with subcommands like `wp post meta`. We also want to add the extra lines for subcommands with subcommands, so I had to add the second condition. Is there a better way to check for a subcommand with subcommands?