Allow running generators for Engine with usage of other application.

After that commit, developers can set ENGINE_PATH in ENGINE/scripts/rails
file and load application's ./script/rails (most of the time it will be
dummy application used for testing). When running ./script/rails g it will
use application to boot up, but then it will use Engine's root and
configuration for generators.

railties/lib/rails/commands.rb

@@ -11,7 +11,17 @@
 command = aliases[command] || command
 
 case command
-when 'generate', 'destroy', 'plugin', 'benchmarker', 'profiler'
+when 'generate', 'destroy', 'plugin'
+  require APP_PATH
+  Rails.application.require_environment!
+
+  if defined?(ENGINE_PATH)
+    engine = Rails.application.railties.engines.find { |r| r.root.to_s == ENGINE_PATH }
+    Rails.application = engine
+  end
+  require "rails/commands/#{command}"
+
+when 'benchmarker', 'profiler'
   require APP_PATH
   Rails.application.require_environment!
   require "rails/commands/#{command}"