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

1 files changed, 18 insertions, 2 deletions

diff --git a/app/services/system_note_service.rb b/app/services/system_note_service.rb
index 0c8446e7c3d..5ccaa5275b7 100644
--- a/app/services/system_note_service.rb
+++ b/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