Add #merge_base_pipeline to MergeRequest

Refers to the pipeline run on target_sha of actual_head_pipeline,
allowing us to compare the head_pipeline for the merged result of
an MR to the commit immediately prior to merging of new code.

app/models/merge_request.rb

@@ -1599,6 +1599,12 @@ class MergeRequest < ApplicationRecord
       .find_by(sha: diff_base_sha, ref: target_branch)
   end
 
+  def merge_base_pipeline
+    @merge_target_pipeline ||= project.ci_pipelines
+      .order(id: :desc)
+      .find_by(sha: actual_head_pipeline.target_sha, ref: target_branch)
+  end
+
   def discussions_rendered_on_frontend?
     true
   end

spec/models/merge_request_spec.rb

@@ -3520,6 +3520,23 @@ RSpec.describe MergeRequest, factory_default: :keep do
     end
   end
 
+  describe '#merge_base_pipeline' do
+    let(:merge_request) do
+      create(:merge_request, :with_merge_request_pipeline)
+    end
+
+    let(:pre_merge_target_pipeline) do
+      create(:ci_pipeline, ref: merge_request.target_branch, sha: merge_request.target_branch_sha)
+    end
+
+    it 'returns a pipeline pointing to a commit on the target ref' do
+      pre_merge_target_pipeline
+      merge_request.update_head_pipeline
+
+      expect(merge_request.merge_base_pipeline).to eq(pre_merge_target_pipeline)
+    end
+  end
+
   describe '#has_commits?' do
     it 'returns true when merge request diff has commits' do
       allow(subject.merge_request_diff).to receive(:commits_count)