Merge branch 'dz-mr-version-compare' into 'master'

Allow compare merge request versions

## What does this MR do?

Add new functionality to the merge request page. It allows you easily compare merge request versions in one click.  

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

## Why was this MR needed?

To improve code review experience. 

## Screenshots (if relevant)

See discussion

## Does this MR meet the acceptance criteria?

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

https://gitlab.com/gitlab-org/gitlab-ce/issues/13570

See merge request !6127

1 files changed, 1 insertions, 0 deletions

diff --git a/CHANGELOG b/CHANGELOG
index 383ccc38c56..879fb98c26f 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -132,6 +132,7 @@ v 8.11.4
   - Fix issue boards leak private label names and descriptions
   - Fix broken gitlab:backup:restore because of bad permissions on repo storage !6098 (Dirk Hörner)
   - Remove gitorious. !5866
+  - Allow compare merge request versions
 
 v 8.11.3
   - Allow system info page to handle case where info is unavailable