Add Enumerable#group_by

Library/Homebrew/extend/enumerable.rb

@@ -0,0 +1,7 @@
+module Enumerable
+  def group_by
+    inject({}) do |h, e|
+      h.fetch(yield(e)) { |k| h[k] = [] } << e; h
+    end
+  end unless method_defined?(:group_by)
+end

Library/Homebrew/global.rb

@@ -4,6 +4,7 @@ require 'extend/pathname'
 require 'extend/ARGV'
 require 'extend/string'
 require 'extend/symbol'
+require 'extend/enumerable'
 require 'utils'
 require 'exceptions'
 require 'set'

Library/Homebrew/test/testing_env.rb

@@ -12,6 +12,7 @@ require 'extend/fileutils'
 require 'extend/pathname'
 require 'extend/string'
 require 'extend/symbol'
+require 'extend/enumerable'
 require 'exceptions'
 require 'utils'
 require 'rbconfig'