Fix MergeToRefService raises Gitlab::Git::CommandError

This commit fixes the problem that can disturb merge train
process.

app/services/merge_requests/merge_to_ref_service.rb

@@ -60,7 +60,7 @@ module MergeRequests
 
     def commit
       repository.merge_to_ref(current_user, source, merge_request, target_ref, commit_message, first_parent_ref)
-    rescue Gitlab::Git::PreReceiveError => error
+    rescue Gitlab::Git::PreReceiveError, Gitlab::Git::CommandError => error
       raise MergeError, error.message
     end
   end

changelogs/unreleased/fix-merge-to-ref-service-raise-command-error.yml

+---
+title: Fix MergeToRefService raises Gitlab::Git::CommandError
+merge_request: 26465
+author:
+type: fixed

ee/spec/services/merge_trains/create_pipeline_service_spec.rb

@@ -118,6 +118,18 @@ describe MergeTrains::CreatePipelineService do
               let(:expected_reason) { '3:Invalid merge source' }
             end
           end
+
+          context 'when there is a conflict on merge ref creation' do
+            before do
+              allow(project.repository).to receive(:merge_to_ref) do
+                raise Gitlab::Git::CommandError.new('Failed to create merge commit')
+              end
+            end
+
+            it_behaves_like 'returns an error' do
+              let(:expected_reason) { 'Failed to create merge commit' }
+            end
+          end
         end
 
         context 'when previous_ref is nil' do

spec/services/merge_requests/merge_to_ref_service_spec.rb

@@ -91,6 +91,17 @@ describe MergeRequests::MergeToRefService do
 
     it_behaves_like 'successfully evaluates pre-condition checks'
 
+    it 'returns an error when Gitlab::Git::CommandError is raised during merge' do
+      allow(project.repository).to receive(:merge_to_ref) do
+        raise Gitlab::Git::CommandError.new('Failed to create merge commit')
+      end
+
+      result = service.execute(merge_request)
+
+      expect(result[:status]).to eq(:error)
+      expect(result[:message]).to eq('Failed to create merge commit')
+    end
+
     context 'commit history comparison with regular MergeService' do
       before do
         # The merge service needs an authorized user while merge-to-ref