Fix "heroku git:clone" to work on any folder

When running "heroku git:clone app-name" outside an app folder, heroku outputs:

> Cloning from app 'app-name'...
>  !    No app specified.
>  !    Run this command from an app folder or specify which app to use with --app APP.

To fix this, we simply need to explictly add the app name by using "heroku git:clone -a app-name".