Merge branch '22773-add-comparison-link-to-system-note' into 'master'

Add link to comparison from system note, update changelog

## What does this MR do?
Adds a link to the comparison between the previous and latest version.

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

## Why was this MR needed?

## Screenshots (if relevant)

![Screen_Shot_2016-10-01_at_3.37.03_AM](/uploads/89b48dc1b2ed868cf5dd85300e6b37a0/Screen_Shot_2016-10-01_at_3.37.03_AM.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
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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 #22773

See merge request !6607

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.13.0 (unreleased)
+  - Add link from system note to compare with previous version
   - Use gitlab-shell v3.6.2 (GIT TRACE logging)
   - Fix centering of custom header logos (Ashley Dumaine)
   - AbstractReferenceFilter caches project_refs on RequestStore when active

app/services/system_note_service.rb

@@ -24,6 +24,7 @@ module SystemNoteService
     body = "Added #{commits_text}:\n\n"
     body << existing_commit_summary(noteable, existing_commits, oldrev)
     body << new_commit_summary(new_commits).join("\n")
+    body << "\n\n[Compare with previous version](#{diff_comparison_url(noteable, project, oldrev)})"
 
     create_note(noteable: noteable, project: project, author: author, note: body)
   end
@@ -254,8 +255,7 @@ module SystemNoteService
   #
   #   "Started branch `201-issue-branch-button`"
   def new_issue_branch(issue, project, author, branch)
-    h = Gitlab::Routing.url_helpers
-    link = h.namespace_project_compare_url(project.namespace, project, from: project.default_branch, to: branch)
+    link = url_helpers.namespace_project_compare_url(project.namespace, project, from: project.default_branch, to: branch)
 
     body = "Started branch [`#{branch}`](#{link})"
     create_note(noteable: issue, project: project, author: author, note: body)
@@ -466,4 +466,20 @@ module SystemNoteService
   def escape_html(text)
     Rack::Utils.escape_html(text)
   end
+
+  def url_helpers
+    @url_helpers ||= Gitlab::Routing.url_helpers
+  end
+
+  def diff_comparison_url(merge_request, project, oldrev)
+    diff_id = merge_request.merge_request_diff.id
+
+    url_helpers.diffs_namespace_project_merge_request_url(
+      project.namespace,
+      project,
+      merge_request.iid,
+      diff_id: diff_id,
+      start_sha: oldrev
+    )
+  end
 end

spec/features/merge_requests/merge_request_versions_spec.rb

 require 'spec_helper'
 
 feature 'Merge Request versions', js: true, feature: true do
+  let(:merge_request) { create(:merge_request, importing: true) }
+  let(:project) { merge_request.source_project }
+
   before do
     login_as :admin
-    merge_request = create(:merge_request, importing: true)
     merge_request.merge_request_diffs.create(head_commit_sha: '6f6d7e7ed97bb5f0054f2b1df789b39ca89b6ff9')
     merge_request.merge_request_diffs.create(head_commit_sha: '5937ac0a7beb003549fc5fd26fc247adbce4a52e')
-    project = merge_request.source_project
     visit diffs_namespace_project_merge_request_path(project.namespace, project, merge_request)
   end
 
@@ -47,6 +48,16 @@ feature 'Merge Request versions', js: true, feature: true do
       end
     end
 
+    it 'has a path with comparison context' do
+      expect(page).to have_current_path diffs_namespace_project_merge_request_path(
+        project.namespace,
+        project,
+        merge_request.iid,
+        diff_id: 2,
+        start_sha: '6f6d7e7ed97bb5f0054f2b1df789b39ca89b6ff9'
+      )
+    end
+
     it 'should have correct value in the compare dropdown' do
       page.within '.mr-version-compare-dropdown' do
         expect(page).to have_content 'version 1'
@@ -61,10 +72,6 @@ feature 'Merge Request versions', js: true, feature: true do
       expect(page).to have_content '4 changed files with 15 additions and 6 deletions'
     end
 
-    it 'show diff between new and old version' do
-      expect(page).to have_content '4 changed files with 15 additions and 6 deletions'
-    end
-
     it 'should return to latest version when "Show latest version" button is clicked' do
       click_link 'Show latest version'
       page.within '.mr-version-dropdown' do

spec/services/system_note_service_spec.rb

@@ -40,6 +40,12 @@ describe SystemNoteService, services: true do
     describe 'note body' do
       let(:note_lines) { subject.note.split("\n").reject(&:blank?) }
 
+      describe 'comparison diff link line' do
+        it 'adds the comparison text' do
+          expect(note_lines[2]).to match "[Compare with previous version]"
+        end
+      end
+
       context 'without existing commits' do
         it 'adds a message header' do
           expect(note_lines[0]).to eq "Added #{new_commits.size} commits:"