Add TreeController and spec

app/models/tree.rb

@@ -1,5 +1,5 @@
 class Tree
-   include Linguist::BlobHelper
+  include Linguist::BlobHelper
   attr_accessor :path, :tree, :project, :ref
 
   delegate :contents,
@@ -14,8 +14,8 @@ class Tree
     to: :tree
 
   def initialize(raw_tree, project, ref = nil, path = nil)
-    @project, @ref, @path = project, ref, path,
-    @tree = if path
+    @project, @ref, @path = project, ref, path
+    @tree = if path.present?
               raw_tree / path.dup.force_encoding('ascii-8bit')
             else
               raw_tree
@@ -26,6 +26,10 @@ class Tree
     tree.is_a?(Grit::Blob)
   end
 
+  def invalid?
+    tree.nil?
+  end
+
   def empty?
     data.blank?
   end