Use source ref for pipeline webhook

When user uses Pipelines for merge requests, the pipeline is a run on
a merge request ref instead of branch ref. However, we should send
source ref as a webhook in order to respect the original behavior.

changelogs/unreleased/use-source-ref-name-in-webhook.yml

+---
+title: Use source ref in pipeline webhook
+merge_request: 28772
+author:
+type: fixed

lib/gitlab/data_builder/pipeline.rb

@@ -19,7 +19,7 @@ module Gitlab
       def hook_attrs(pipeline)
         {
           id: pipeline.id,
-          ref: pipeline.ref,
+          ref: pipeline.source_ref,
           tag: pipeline.tag,
           sha: pipeline.sha,
           before_sha: pipeline.before_sha,

spec/lib/gitlab/data_builder/pipeline_spec.rb

@@ -50,5 +50,14 @@ describe Gitlab::DataBuilder::Pipeline do
       it { expect(attributes[:variables]).to be_a(Array) }
       it { expect(attributes[:variables]).to contain_exactly({ key: 'TRIGGER_KEY_1', value: 'TRIGGER_VALUE_1' }) }
     end
+
+    context 'when pipeline is a detached merge request pipeline' do
+      let(:merge_request) { create(:merge_request, :with_detached_merge_request_pipeline) }
+      let(:pipeline) { merge_request.all_pipelines.first }
+
+      it 'returns a source ref' do
+        expect(attributes[:ref]).to eq(merge_request.source_branch)
+      end
+    end
   end
 end