Fix download link for huge MR diffs

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

CHANGELOG

@@ -14,6 +14,7 @@ v 6.8.0
   - Fix faulty namespace names that caused 500 on user creation
   - Option to disable standard login
   - Clean old created archives from repository downloads directory
+  - Fix download link for huge MR diffs
 v 6.7.3
   - Fix the merge notification email not being sent (Pierre de La Morinerie)

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

@@ -8,5 +8,5 @@
       Changes view 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"
+      = link_to "download it", project_merge_request_path(@merge_request.target_project, @merge_request, format: :diff), class: "vlink"
       instead.