add --http-git option to create, git:clone and git:remote

Still working on proper manual testing, so don't put too much faith into this yet.

This will break if someone sets [`HEROKU_GIT_HOST`](https://github.com/heroku/heroku/blob/master/lib/heroku/auth.rb#L56-L58). Seems like `Heroku::Auth.git_host` should be resolving the HTTP vs SSH differences here to allow the override.