FIX: Add topic_diff to PostRevisor (#12518)

The instance of the PostRevisor is passed to the post_edited
event. It is useful to know what has happened to the topic in
this event (we already pass a boolean for topic_changed? but that
is not so helpful by itself).

lib/post_revisor.rb

@@ -536,6 +536,10 @@ class PostRevisor
     @post.previous_changes.slice(*POST_TRACKED_FIELDS)
   end
 
+  def topic_diff
+    @topic_changes.diff
+  end
+
   def perform_edit
     return if bypass_rate_limiter?
     EditRateLimiter.new(@editor).performed!