Fix MWPS system notes shows inconsistent sha

Fix the system note service

app/services/auto_merge/merge_when_pipeline_succeeds_service.rb

@@ -5,7 +5,7 @@ module AutoMerge
     def execute(merge_request)
       super do
         if merge_request.saved_change_to_auto_merge_enabled?
-          SystemNoteService.merge_when_pipeline_succeeds(merge_request, project, current_user, merge_request.diff_head_commit)
+          SystemNoteService.merge_when_pipeline_succeeds(merge_request, project, current_user, merge_request.actual_head_pipeline.sha)
         end
       end
     end

app/services/system_note_service.rb

@@ -221,8 +221,8 @@ module SystemNoteService
   end
 
   # Called when 'merge when pipeline succeeds' is executed
-  def merge_when_pipeline_succeeds(noteable, project, author, last_commit)
-    body = "enabled an automatic merge when the pipeline for #{last_commit.to_reference(project)} succeeds"
+  def merge_when_pipeline_succeeds(noteable, project, author, sha)
+    body = "enabled an automatic merge when the pipeline for #{sha} succeeds"
 
     create_note(NoteSummary.new(noteable, project, author, body, action: 'merge'))
   end

spec/services/auto_merge/base_service_spec.rb

@@ -59,6 +59,11 @@ describe AutoMerge::BaseService do
     context 'when strategy is merge when pipeline succeeds' do
       let(:service) { AutoMerge::MergeWhenPipelineSucceedsService.new(project, user) }
 
+      before do
+        pipeline = build(:ci_pipeline)
+        allow(merge_request).to receive(:actual_head_pipeline) { pipeline }
+      end
+
       it 'sets the auto merge strategy' do
         subject
 

spec/services/auto_merge/merge_when_pipeline_succeeds_service_spec.rb

@@ -64,8 +64,11 @@ describe AutoMerge::MergeWhenPipelineSucceedsService do
       end
 
       it 'creates a system note' do
+        pipeline = build(:ci_pipeline)
+        allow(merge_request).to receive(:actual_head_pipeline) { pipeline }
+
         note = merge_request.notes.last
-        expect(note.note).to match %r{enabled an automatic merge when the pipeline for (\w+/\w+@)?\h{8}}
+        expect(note.note).to match "enabled an automatic merge when the pipeline for #{pipeline.sha}"
       end
     end
 

spec/services/system_note_service_spec.rb

@@ -332,7 +332,7 @@ describe SystemNoteService do
       create(:merge_request, source_project: project, target_project: project)
     end
 
-    subject { described_class.merge_when_pipeline_succeeds(noteable, project, author, noteable.diff_head_commit) }
+    subject { described_class.merge_when_pipeline_succeeds(noteable, project, author, pipeline.sha) }
 
     it_behaves_like 'a system note' do
       let(:action) { 'merge' }