Fetch reference from fork repo when create merge request

app/models/repository.rb

@@ -465,6 +465,11 @@ class Repository
     )
   end
 
+  def fetch_ref(source_path, source_ref, target_ref)
+    args = %W(git fetch #{source_path} #{source_ref}:#{target_ref})
+    Gitlab::Popen.popen(args, path_to_repo)
+  end
+
   private
 
   def cache

app/services/merge_requests/create_service.rb

@@ -9,6 +9,15 @@ module MergeRequests
       merge_request.author = current_user
 
       if merge_request.save
+        # Fetch fork branch into hidden ref of target repository
+        if merge_request.for_fork?
+          merge_request.target_project.repository.fetch_ref(
+            merge_request.source_project.repository.path_to_repo,
+            "refs/heads/#{merge_request.source_branch}",
+            "refs/merge-requests/#{merge_request.id}/head"
+          )
+        end
+
         merge_request.update_attributes(label_ids: label_params)
         event_service.open_mr(merge_request, current_user)
         notification_service.new_merge_request(merge_request, current_user)