Rework diff slightly

Rakefile

@@ -26,6 +26,7 @@ end
 namespace :benchmark do
   require 'git'
   require 'linguist/language'
+  require './lib/linguist/diff'
 
   git = Git.open('.')
 

lib/linguist/diff.rb

@@ -1,4 +1,4 @@
-class Hash
+module HashDiff
   def deep_diff(b)
     a = self
     (a.keys | b.keys).inject({}) do |diff, k|
@@ -13,3 +13,7 @@ class Hash
     end
   end
 end
+
+class Hash
+  include HashDiff
+end