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

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

> Is there a better way to check for a subcommand with subcommands?

Nothing off the top of my head. If not, it would be nice to have one (with some unit test coverage).

Related: we really should have a glossary of all these terms.