Merge branch 'update-hint-for-local-merge' into 'master'

Updates hint text for merging branches locally.

Closes #39398

See merge request gitlab-org/gitlab-ce!21421

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

@@ -30,11 +30,13 @@
         %pre.dark#merge-info-3
           - if @merge_request.for_fork?
             :preserve
-              git checkout #{h @merge_request.target_branch}
+              git fetch origin
+              git checkout origin/#{h @merge_request.target_branch}
               git merge --no-ff #{h @merge_request.source_project_path}-#{h @merge_request.source_branch}
           - else
             :preserve
-              git checkout #{h @merge_request.target_branch}
+              git fetch origin
+              git checkout origin/#{h @merge_request.target_branch}
               git merge --no-ff #{h @merge_request.source_branch}
         %p
           %strong Step 4.