Increases width of modalbox with checkout branch instructions

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

 #modal_merge_info.modal{ tabindex: '-1' }
-  .modal-dialog
+  .modal-dialog.modal-lg
     .modal-content
       .modal-header
         %h3.modal-title Check out, review, and merge locally

changelogs/unreleased/43625-increase-modal-checkout.yml

+---
+title: Increase width of checkout branch modal box
+merge_request:
+author:
+type: fixed