Add a spec for diff
user
```ruby
require File.dirname(__FILE__) + '/integration_helper'

describe 'Running braid diff on a mirror' do
  before do
    FileUtils.rm_rf(TMP_PATH)
    FileUtils.mkdir_p(TMP_PATH)
    @repository_dir = create_git_repo_from_fixture('shiny')
    @vendor_repository_dir = create_git_repo_from_fixture('skit1')
  end

  describe 'braided directly in' do
    before do
      in_dir(@repository_dir) do
        run_command("#{BRAID_BIN} add #{@vendor_repository_dir}")
      end
    end
    describe 'with no changes' do

      it 'should emit no output when named in braid diff' do
        diff = nil
        in_dir(@repository_dir) do
          diff = run_command("#{BRAID_BIN} diff skit1")
        end

        expect(diff).to eq('')
      end

      it 'should emit only banners when braid diff all' do
        diff = nil
        in_dir(@repository_dir) do
          diff = run_command("#{BRAID_BIN} diff")
        end

        expect(diff).to eq("=======================================================\nBraid: Diffing skit1\n=======================================================\n")
      end
    end
  end
end
```
assistant