Add test spec for TreeHelper module

spec/helpers/tree_helper_spec.rb

+require 'spec_helper'
+
+describe TreeHelper do
+  describe 'flatten_tree' do
+    let(:project) { create(:project) }
+
+    before {
+      @repository = project.repository
+      @commit = project.repository.commit
+    }
+
+    context "on a directory containing more than one file/directory" do
+      let(:tree_item) { double(name: "files", path: "files") }
+
+      it "should return the directory name" do
+        flatten_tree(tree_item).should match('files')
+      end
+    end
+
+    context "on a directory containing only one directory" do
+      let(:tree_item) { double(name: "foo", path: "foo") }
+
+      it "should return the flattened path" do
+        flatten_tree(tree_item).should match('foo/bar')
+      end
+    end
+  end
+end