Change "check out, review, and merge locally" URL to use default clone protocol.

Using the default protocol here (vs HTTP only) is more convenient
when it comes to pasting these commands into a terminal.

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