Merge branch 'change-checkout-locally-url' into 'master'

Change "check out, review, and merge locally" URL to SSH by default.

## What does this MR do?

Change the default URL type  in the "Check out, review, and merge locally" pop-up dialog to SSH.

## Why was this MR needed?

The default URL to show on the project screen is the SSH URL, so it makes sense for it to be the default here too. Plus, since most people (hopefully) use SSH keys to avoid typing credentials repeatedly, changing this default means these commands can be pasted into a terminal more easily since no password prompt will get in the way.

## Does this MR meet the acceptance criteria?

- [X] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [n/a ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [n/a] API support added
- Tests
  - [n/a] 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)

See merge request !6211

CHANGELOG

@@ -89,6 +89,7 @@ v 8.12.0 (unreleased)
   - Fix error on raw build trace download for old builds stored in database !4822
   - Refactor the triggers page and documentation !6217
   - Show values of CI trigger variables only when clicked (Katarzyna Kobierska Ula Budziszewska)
+  - Use default clone protocol on "check out, review, and merge locally" help page URL
 
 v 8.11.5 (unreleased)
   - Optimize branch lookups and force a repository reload for Repository#find_branch

app/views/projects/merge_requests/show/_how_to_merge.html.haml

@@ -12,7 +12,7 @@
         %pre.dark#merge-info-1
           - if @merge_request.for_fork?
             :preserve
-              git fetch #{h @merge_request.source_project.http_url_to_repo} #{h @merge_request.source_branch}
+              git fetch #{h default_url_to_repo(@merge_request.source_project)} #{h @merge_request.source_branch}
               git checkout -b #{h @merge_request.source_project_path}-#{h @merge_request.source_branch} FETCH_HEAD
           - else
             :preserve