Fix link for MR diff download

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

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

@@ -3,8 +3,10 @@
 - elsif @merge_request_diff.empty?
   %h4.nothing_here_message Nothing to merge from #{@merge_request.source_branch} into #{@merge_request.target_branch}
 - else
-  %h4.nothing_here_message
-    Can't load diff.
-    You can
-    = link_to "download it", project_merge_request_path(@merge_request.source_project, @merge_request), format: :diff, class: "vlink"
-    instead.
+  .bs-callout.bs-callout-warning
+    %h4
+      Diff for this comparison is extremely large.
+    %p
+      You can
+      = link_to "download it", project_merge_request_path(@merge_request.source_project, @merge_request, format: :diff), class: "vlink"
+      instead.