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, 9 insertions, 0 deletions

diff --git a/spec/helpers/git_helper_spec.rb b/spec/helpers/git_helper_spec.rb
new file mode 100644
index 00000000000..9b1ef1e05a2
--- /dev/null
+++ b/spec/helpers/git_helper_spec.rb
@@ -0,0 +1,9 @@
+require 'spec_helper'
+
+describe GitHelper do
+  describe '#short_sha' do
+    let(:short_sha) { helper.short_sha('d4e043f6c20749a3ab3f4b8e23f2a8979f4b9100') }
+
+    it { expect(short_sha).to eq('d4e043f6') }
+  end
+end