Add spec for compare service

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

spec/services/compare_service_spec.rb

+require 'spec_helper'
+
+describe CompareService, services: true do
+  let(:project) { create(:project) }
+  let(:user) { create(:user) }
+  let(:service) { described_class.new }
+
+  describe '#execute' do
+    context 'compare with base, like feature...fix' do
+      subject { service.execute(project, 'feature', project, 'fix', false) }
+
+      it { expect(subject.diffs.size).to eq(1) }
+    end
+
+    context 'straight compare, like feature..fix' do
+      subject { service.execute(project, 'feature', project, 'fix', true) }
+
+      it { expect(subject.diffs.size).to eq(3) }
+    end
+  end
+end