Remove support for branch-less builds in MWBS

See !6675#note_16580143

app/services/merge_requests/base_service.rb

@@ -42,20 +42,19 @@ module MergeRequests
       super(:merge_request)
     end
 
-    def merge_requests_for(branches, sha)
-      # This is for ref-less builds
-      branches ||= @project.repository.branch_names_contains(sha)
+    def merge_requests_for(branch)
+      origin_merge_requests = @project.origin_merge_requests
+        .opened.where(source_branch: branch).to_a
 
-      return [] if branches.blank?
+      fork_merge_requests = @project.fork_merge_requests
+        .opened.where(source_branch: branch).to_a
 
-      merge_requests = @project.origin_merge_requests.opened.where(source_branch: branches).to_a
-      merge_requests += @project.fork_merge_requests.opened.where(source_branch: branches).to_a
-
-      merge_requests.uniq.select(&:source_project)
+      (origin_merge_requests + fork_merge_requests)
+        .uniq.select(&:source_project)
     end
 
     def pipeline_merge_requests(pipeline)
-      merge_requests_for(pipeline.ref, pipeline.sha).each do |merge_request|
+      merge_requests_for(pipeline.ref).each do |merge_request|
         next unless pipeline == merge_request.pipeline
 
         yield merge_request
@@ -63,8 +62,9 @@ module MergeRequests
     end
 
     def commit_status_merge_requests(commit_status)
-      merge_requests_for(commit_status.ref, commit_status.sha).each do |merge_request|
+      merge_requests_for(commit_status.ref).each do |merge_request|
         pipeline = merge_request.pipeline
+
         next unless pipeline
         next unless pipeline.sha == commit_status.sha
 

spec/services/merge_requests/merge_when_build_succeeds_service_spec.rb

@@ -97,20 +97,9 @@ describe MergeRequests::MergeWhenBuildSucceedsService do
         expect(MergeWorker).not_to receive(:perform_async)
         service.trigger(unrelated_pipeline)
       end
-
-      it 'discovers branches and merges all merge requests when status is success' do
-        allow(project.repository).to receive(:branch_names_contains).
-          with(commit_status.sha).and_return([mr_merge_if_green_enabled.source_branch])
-        allow(pipeline).to receive(:success?).and_return(true)
-        allow_any_instance_of(MergeRequest).to receive(:pipeline).and_return(pipeline)
-        allow(pipeline).to receive(:success?).and_return(true)
-
-        expect(MergeWorker).to receive(:perform_async)
-        service.trigger(commit_status)
-      end
     end
 
-    context 'properly handles multiple stages' do
+    context 'when there are multiple stages in the pipeline' do
       let(:ref) { mr_merge_if_green_enabled.source_branch }
       let!(:build) { create(:ci_build, :created, pipeline: pipeline, ref: ref, name: 'build', stage: 'build') }
       let!(:test) { create(:ci_build, :created, pipeline: pipeline, ref: ref, name: 'test', stage: 'test') }