Improve post_receive spec by not stubbing private methods

spec/workers/post_receive_spec.rb

@@ -66,19 +66,21 @@ describe PostReceive do
     end
 
     context "gitlab-ci.yml" do
+      let(:changes) { "123456 789012 refs/heads/feature\n654321 210987 refs/tags/tag" }
+
       subject { described_class.new.perform(gl_repository, key_id, base64_changes) }
 
       context "creates a Ci::Pipeline for every change" do
         before do
           stub_ci_pipeline_to_return_yaml_file
 
-          # TODO, don't stub private methods
-          #
-          allow_any_instance_of(Ci::CreatePipelineService)
-            .to receive(:commit).and_return(OpenStruct.new(id: '123456'))
+          allow_any_instance_of(Project)
+            .to receive(:commit)
+            .and_return(project.commit)
 
           allow_any_instance_of(Repository)
-            .to receive(:branch_exists?).and_return(true)
+            .to receive(:branch_exists?)
+            .and_return(true)
         end
 
         it { expect { subject }.to change { Ci::Pipeline.count }.by(2) }