Gracefully handle notes on deleted commits in merge requests

Closes #3250

CHANGELOG

@@ -8,6 +8,7 @@ v 8.7.0 (unreleased)
   - Add links to CI setup documentation from project settings and builds pages
   - Implement 'Groups View' as an option for dashboard preferences !3379 (Elias W.)
   - Implement 'TODOs View' as an option for dashboard preferences !3379 (Elias W.)
+  - Gracefully handle notes on deleted commits in merge requests (Stan Hu)
 
 v 8.6.2 (unreleased)
   - Comments on confidential issues don't show up in activity feed to non-members

app/helpers/notes_helper.rb

@@ -5,9 +5,11 @@ module NotesHelper
   end
 
   def note_target_fields(note)
+    if note.noteable
       hidden_field_tag(:target_type, note.noteable.class.name.underscore) +
         hidden_field_tag(:target_id, note.noteable.id)
     end
+  end
 
   def note_editable?(note)
     note.editable? && can?(current_user, :admin_note, note)

app/views/projects/notes/discussions/_commit.html.haml

 - note = discussion_notes.first
+- commit = note.noteable
+- commit_description = commit ? 'commit' : 'a deleted commit'
 .discussion.js-toggle-container{ class: note.discussion_id }
   .discussion-header
     .discussion-actions
@@ -7,8 +9,9 @@
         Show/hide discussion
     %div
       = link_to_member(@project, note.author, avatar: false)
-      started a discussion on commit
-      = link_to(note.noteable.short_id, namespace_project_commit_path(note.project.namespace, note.project, note.noteable), class: 'monospace')
+      %p started a discussion on #{commit_description}
+      - if commit
+        = link_to(commit.short_id, namespace_project_commit_path(note.project.namespace, note.project, note.noteable), class: 'monospace')
     .last-update.hide.js-toggle-content
       - last_note = discussion_notes.last
       last updated by