Use a more future-proof check for Note/LegacyDiffNote

Signed-off-by: Rémy Coutable <remy@rymai.me>

app/models/event.rb

@@ -136,7 +136,7 @@ class Event < ActiveRecord::Base
   end
 
   def note?
-    %w[Note LegacyDiffNote].include?(target_type)
+    target.is_a?(Note)
   end
 
   def issue?

spec/models/event_spec.rb

@@ -55,8 +55,8 @@ describe Event, models: true do
       it { is_expected.to be_note }
     end
 
-    context 'merge request note event' do
-      let(:target) { create(:note_on_merge_request) }
+    context 'merge request diff note event' do
+      let(:target) { create(:note_on_merge_request_diff) }
 
       it { is_expected.to be_note }
     end
@@ -129,10 +129,10 @@ describe Event, models: true do
       end
     end
 
-    context 'merge request note event' do
+    context 'merge request diff note event' do
       let(:project) { create(:project, :public) }
       let(:merge_request) { create(:merge_request, source_project: project, author: author, assignee: assignee) }
-      let(:note_on_merge_request) { create(:note_on_merge_request, noteable: merge_request, project: project) }
+      let(:note_on_merge_request) { create(:note_on_merge_request_diff, noteable: merge_request, project: project) }
       let(:target) { note_on_merge_request }
 
       it { expect(event.visible_to_user?(non_member)).to eq true }