script/plugin install -b BRANCH_OR_TAG git:....

This patch was inspired by multiple RSpec tag installs (RSpec edge
works with the existing script/plugin).

From 2f47df36bc3037ccd978e9b462e654558c498286 Mon Sep 17 00:00:00 2001

Opened ticket #591

http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/591-installing-plugin-from-git-branches