HOWTO setup multistage deployment with Capistrano

These instructions are outdated. They were written for Capistrano 2.x. As of March 2016, Capistrano 3.x is current. When I wrote about deploying PHP sites with Capistrano, step 6 was a bit of a throwaway — hey, here's a quick hack for handling multistage deployment. I'd been using variants of that technique for quite some …