Simplify MergedCommentTree#initialize

By setting the comments lazily in a different method, we can share its
code with the parent class.

lib/comment_tree.rb

@@ -2,14 +2,17 @@ class CommentTree
 
   ROOT_COMMENTS_PER_PAGE = 10
 
-  attr_accessor :root_comments, :comments, :commentable, :page, :order
+  attr_reader :root_comments, :commentable, :page, :order
 
   def initialize(commentable, page, order = "confidence_score", valuations: false)
     @commentable = commentable
     @page = page
     @order = order
     @valuations = valuations
-    @comments = root_comments + root_descendants
+  end
+
+  def comments
+    @comments ||= root_comments + root_descendants
   end
 
   def root_comments