Merge branch '21535-fix-hidden-diff-comments' into 'master'

Resolve "Resolved comments permanently hidden in Side-by-Side diff view"

## What does this MR do? / Why was this MR needed?

Ensures resolved discussions are made visible when clicking "toggle comments" on Side-by-Side view diff pages.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #21535

See merge request !6575

CHANGELOG

@@ -13,6 +13,7 @@ v 8.13.0 (unreleased)
   - Revoke button in Applications Settings underlines on hover.
   - Update ruby-prof to 0.16.2. !6026 (Elan Ruusamäe)
   - Add organization field to user profile
+  - Fix resolved discussion display in side-by-side diff view !6575
   - Optimize GitHub importing for speed and memory
   - API: expose pipeline data in builds API (!6502, Guilherme Salazar)
 

app/assets/javascripts/application.js

@@ -247,7 +247,7 @@
       $this.toggleClass('active');
       var notesHolders = $this.closest('.diff-file').find('.notes_holder');
       if ($this.hasClass('active')) {
-        notesHolders.show();
+        notesHolders.show().find('.hide').show();
       } else {
         notesHolders.hide();
       }

app/views/projects/diffs/_file.html.haml

@@ -5,7 +5,7 @@
     - unless diff_file.submodule?
       .file-actions.hidden-xs
         - if blob_text_viewable?(blob)
-          = link_to '#', class: 'js-toggle-diff-comments btn active has-tooltip btn-file-option', title: "Toggle comments for this files", disabled: @diff_notes_disabled do
+          = link_to '#', class: 'js-toggle-diff-comments btn active has-tooltip btn-file-option', title: "Toggle comments for this file", disabled: @diff_notes_disabled do
             = icon('comment')
           \