Fix label links in MR system notes

This changes label change system notes in MRs to link to the merge
request list instead of the issue list.

Changelog: fixed

app/models/resource_label_event.rb

@@ -54,7 +54,7 @@ class ResourceLabelEvent < ResourceEvent
   end
 
   def banzai_render_context(field)
-    super.merge(pipeline: :label, only_path: true)
+    super.merge(pipeline: :label, only_path: true, label_url_method: label_url_method)
   end
 
   def refresh_invalid_reference
@@ -91,6 +91,10 @@ class ResourceLabelEvent < ResourceEvent
     end
   end
 
+  def label_url_method
+    issuable.is_a?(MergeRequest) ? :project_merge_requests_url : :project_issues_url
+  end
+
   def expire_etag_cache
     issuable.expire_note_etag_cache
   end

ee/app/models/ee/resource_label_event.rb

@@ -15,19 +15,23 @@ module EE
       end
     end
 
-    override :discussion_id_key
-    def discussion_id_key
-      (super + [label&.scoped_label?]).compact # label can be nil as we don't remove resource events when deleting labels
-    end
-
     override :issuable
     def issuable
       epic || super
     end
 
-    override :banzai_render_context
-    def banzai_render_context(field)
-      epic ? super.merge(label_url_method: :group_epics_url) : super
+    private
+
+    override :label_url_method
+    def label_url_method
+      return :group_epics_url if issuable.is_a?(Epic)
+
+      super
+    end
+
+    override :discussion_id_key
+    def discussion_id_key
+      (super + [label&.scoped_label?]).compact # label can be nil as we don't remove resource events when deleting labels
     end
   end
 end

spec/models/label_note_spec.rb

@@ -3,6 +3,8 @@
 require 'spec_helper'
 
 RSpec.describe LabelNote do
+  include Gitlab::Routing.url_helpers
+
   let_it_be(:project) { create(:project, :repository) }
   let_it_be(:user) { create(:user) }
   let_it_be(:label) { create(:label, project: project) }
@@ -14,11 +16,27 @@ RSpec.describe LabelNote do
     let_it_be(:resource) { create(:issue, project: project) }
 
     it_behaves_like 'label note created from events'
+
+    it 'includes a link to the list of issues filtered by the label' do
+      note = described_class.from_events([
+        create(:resource_label_event, label: label, issue: resource)
+      ])
+
+      expect(note.note_html).to include(project_issues_path(project, label_name: label.title))
+    end
   end
 
   context 'when resource is merge request' do
     let_it_be(:resource) { create(:merge_request, source_project: project, target_project: project) }
 
     it_behaves_like 'label note created from events'
+
+    it 'includes a link to the list of merge requests filtered by the label' do
+      note = described_class.from_events([
+        create(:resource_label_event, label: label, merge_request: resource)
+      ])
+
+      expect(note.note_html).to include(project_merge_requests_path(project, label_name: label.title))
+    end
   end
 end