Merge branch 'toggle-whitespace-button-for-compare-page' into 'master'

Toggle whitespace button for compare page

## What does this MR do?

Adds whitespace button to compare branches page.

## Are there points in the code the reviewer needs to double check?

Ruby code.

## Why was this MR needed?

Compare branches UX

## What are the relevant issue numbers?

Closes #17881.

## Screenshots (if relevant)

![Screen_Shot_2016-06-09_at_18.39.48](/uploads/8995a7bc8bea7fa3ff19446ebe49fb88/Screen_Shot_2016-06-09_at_18.39.48.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- [ ] Tests
  - [ ] Added for this feature/bug
  - [ ] 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)

See merge request !4569

CHANGELOG

@@ -71,6 +71,7 @@ v 8.9.0 (unreleased)
   - Add support for using Yubikeys (U2F) for two-factor authentication
   - Link to blank group icon doesn't throw a 404 anymore
   - Remove 'main language' feature
+  - Toggle whitespace button now available for compare branches diffs #17881
   - Pipelines can be canceled only when there are running builds
   - Allow authentication using personal access tokens
   - Use downcased path to container repository as this is expected path by Docker

app/helpers/diff_helper.rb

@@ -135,6 +135,11 @@ module DiffHelper
     toggle_whitespace_link(url, options)
   end
 
+  def diff_compare_whitespace_link(project, from, to, options)
+    url = namespace_project_compare_path(project.namespace, project, from, to, params_with_whitespace)
+    toggle_whitespace_link(url, options)
+  end
+
   private
 
   def hide_whitespace?

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

@@ -11,6 +11,8 @@
         = commit_diff_whitespace_link(@project, @commit, class: 'hidden-xs')
       - elsif current_controller?(:merge_requests)
         = diff_merge_request_whitespace_link(@project, @merge_request, class: 'hidden-xs')
+      - elsif current_controller?(:compare)
+        = diff_compare_whitespace_link(@project, params[:from], params[:to], class: 'hidden-xs')
     .btn-group
       = inline_diff_btn
       = parallel_diff_btn