Fix Epic bulk updates leaking to other epics

When doing a bulk update on epics, the set of labels for the first
selected epic are copied over to the rest of the epics.

This is because we compute the final list of labels and sets it to
`params` and this `params` is the same one used for updating other
epics.

Changelog: fixed
EE: true

app/services/issuable/bulk_update_service.rb

@@ -57,7 +57,11 @@ module Issuable
       items.each do |issuable|
         next unless can?(current_user, :"update_#{type}", issuable)
 
-        update_class.new(**update_class.constructor_container_arg(issuable.issuing_parent), current_user: current_user, params: params).execute(issuable)
+        update_class.new(
+          **update_class.constructor_container_arg(issuable.issuing_parent),
+          current_user: current_user,
+          params: params.dup
+        ).execute(issuable)
       end
 
       items

ee/spec/services/ee/issuable/bulk_update_service_spec.rb

@@ -203,6 +203,19 @@ RSpec.describe Issuable::BulkUpdateService do
         let(:new_value) { [label2, label3] }
 
         it_behaves_like 'updates issuables attribute', :labels
+
+        context 'when epics have different labels' do
+          let(:label4) { create(:group_label, group: group, title: 'feature') }
+          let(:epic1) { create(:epic, group: group, labels: [label1, label4]) }
+
+          it 'keeps existing labels' do
+            expect(subject.success?).to be_truthy
+            expect(subject.payload[:count]).to eq(issuables.count)
+
+            expect(epic1.reload.labels).to contain_exactly(label4, label2, label3)
+            expect(epic2.reload.labels).to contain_exactly(label2, label3)
+          end
+        end
       end
 
       context 'when epics are disabled' do