Merge branch 'fix/merge-request-close-message' into 'master'

Prepend blank line to close message on merge request

## What does this MR do?
Adds a new line above the "Closes" message on a merge request.

## Why was this MR needed?
Autoappended `Closes` to the end of merge request needs a new line in the beginning. When using descriptive commit messages and the commit message body is used for the body of the merge request the Closes message should appear on a line by itself.

## 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
  - [x] 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?
#21710

See merge request !6151

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.12.0 (unreleased)
+  - Prepend blank line to `Closes` message on merge request linked to issue (lukehowell)
   - Filter tags by name !6121
   - Make push events have equal vertical spacing.
   - Add two-factor recovery endpoint to internal API !5510

app/services/merge_requests/build_service.rb

@@ -83,7 +83,7 @@ module MergeRequests
         closes_issue = "Closes ##{iid}"
 
         if merge_request.description.present?
-          merge_request.description += closes_issue.prepend("\n")
+          merge_request.description += closes_issue.prepend("\n\n")
         else
           merge_request.description = closes_issue
         end

spec/services/merge_requests/build_service_spec.rb

@@ -99,14 +99,14 @@ describe MergeRequests::BuildService, services: true do
         let(:source_branch) { "#{issue.iid}-fix-issue" }
 
         it 'appends "Closes #$issue-iid" to the description' do
-          expect(merge_request.description).to eq("#{commit_1.safe_message.split(/\n+/, 2).last}\nCloses ##{issue.iid}")
+          expect(merge_request.description).to eq("#{commit_1.safe_message.split(/\n+/, 2).last}\n\nCloses ##{issue.iid}")
         end
 
         context 'merge request already has a description set' do
           let(:description) { 'Merge request description' }
 
           it 'appends "Closes #$issue-iid" to the description' do
-            expect(merge_request.description).to eq("#{description}\nCloses ##{issue.iid}")
+            expect(merge_request.description).to eq("#{description}\n\nCloses ##{issue.iid}")
           end
         end