PERF: Memoize PostRevisor#diff_size (#8939)

Although this method is usually called twice, its execution can take
about 1s.

lib/post_revisor.rb

@@ -287,8 +287,10 @@ class PostRevisor
   end
 
   def diff_size(before, after)
-    ONPDiff.new(before, after).short_diff.sum do |str, type|
-      type == :common ? 0 : str.size
+    @diff_size ||= begin
+      ONPDiff.new(before, after).short_diff.sum do |str, type|
+        type == :common ? 0 : str.size
+      end
     end
   end