Add a title attribute to each branch in the branches dropdown on the New MR page.

app/views/projects/merge_requests/dropdowns/_branch.html.haml

 %ul
   - branches.each do |branch|
     %li
-      %a{ href: '#', class: "#{('is-active' if selected == branch)}", data: { id: branch } }
+      %a{ href: '#', class: "#{('is-active' if selected == branch)}", title: branch, data: { id: branch } }
         = branch