Add spec for Repository#ff_merge (ported from EE)

spec/models/repository_spec.rb

@@ -1308,6 +1308,34 @@ describe Repository, models: true do
     end
   end
 
+  describe '#ff_merge' do
+    before do
+      repository.add_branch(user, 'ff-target', 'feature~5')
+    end
+
+    it 'merges the code and return the commit id' do
+      merge_request = create(:merge_request, source_branch: 'feature', target_branch: 'ff-target', source_project: project)
+      merge_commit_id = repository.ff_merge(user,
+                                            merge_request.diff_head_sha,
+                                            merge_request.target_branch,
+                                            merge_request: merge_request)
+      merge_commit = repository.commit(merge_commit_id)
+
+      expect(merge_commit).to be_present
+      expect(repository.blob_at(merge_commit.id, 'files/ruby/feature.rb')).to be_present
+    end
+
+    it 'sets the `in_progress_merge_commit_sha` flag for the given merge request' do
+      merge_request = create(:merge_request, source_branch: 'feature', target_branch: 'ff-target', source_project: project)
+      merge_commit_id = repository.ff_merge(user,
+                                            merge_request.diff_head_sha,
+                                            merge_request.target_branch,
+                                            merge_request: merge_request)
+
+      expect(merge_request.in_progress_merge_commit_sha).to eq(merge_commit_id)
+    end
+  end
+
   describe '#revert' do
     let(:new_image_commit) { repository.commit('33f3729a45c02fc67d00adb1b8bca394b0e761d9') }
     let(:update_image_commit) { repository.commit('2f63565e7aac07bcdadb654e253078b727143ec4') }

spec/serializers/merge_request_entity_spec.rb

@@ -47,7 +47,8 @@ describe MergeRequestEntity do
                                :cancel_merge_when_pipeline_succeeds_path,
                                :create_issue_to_resolve_discussions_path,
                                :source_branch_path, :target_branch_commits_path,
-                               :target_branch_tree_path, :commits_count, :merge_ongoing)
+                               :target_branch_tree_path, :commits_count, :merge_ongoing,
+                               :ff_only_enabled)
   end
 
   it 'has email_patches_path' do