Add UI for custom Merge request commit message

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/projects/merge_requests/show/_mr_accept.html.haml

@@ -13,7 +13,21 @@
           If you still want to do it manually -
           %strong
             = link_to "click here", "#modal_merge_info", class: "how_to_merge_link vlink", title: "How To Merge", "data-toggle" => "modal"
-          for instructions
+          for instructions.
+
+          %br
+          If you want to modify merge commit message -
+          %strong
+            = link_to "click here", "#", class: "js-toggle-visibility-link", title: "Modify merge commit message"
+
+        .js-toggle-visibility-container.hide
+          .form-group
+            = label_tag :merge_commit_message, "Commit message", class: 'control-label'
+            .col-sm-10
+              = text_area_tag :merge_commit_message, @merge_request.merge_commit_message, class: "form-control js-gfm-input", rows: 14, required: true
+              %p.hint
+                The recommended maximum line length is 52 characters for the first line and 72 characters for all following lines.
+
         .accept-group
           .pull-left
             = f.submit "Accept Merge Request", class: "btn btn-create accept_merge_request"