Merge branch 'rs-issue-15408' into 'master'

Fix undefined local variable error in Invalid MR template

Closes #15408

See merge request !3857

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

-- page_title "#{@merge_request.title} (#{merge_request.to_reference}", "Merge Requests"
+- page_title "#{@merge_request.title} (#{@merge_request.to_reference}", "Merge Requests"
 = render "header_title"
 
 .merge-request