Copy LFS objects when a repository is pull mirrored

What does this MR do?

Copies LFS objects into the repository when it is pull mirrored.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/10871

ee/spec/services/projects/update_mirror_service_spec.rb

@@ -229,12 +229,12 @@ def create_file(repository)
 
         context 'when Lfs import fails' do
           it 'the mirror operation fails' do
-            error_message = 'error message'
-            expect_any_instance_of(Projects::LfsPointers::LfsImportService).to receive(:execute).and_return(status: :error, message: error_message)
+            expect_any_instance_of(Projects::LfsPointers::LfsImportService).to receive(:execute).and_return(status: :error, message: 'error message')
 
             result = subject.execute
+
             expect(result[:status]).to eq :error
-            expect(result[:message]).to eq error_message
+            expect(result[:message]).to eq 'error message'
           end
         end
       end