Link correct project when adding file to empty MR

When creating a cross project merge request without changes. The MR
widget shows the option to create a new file. It would however link to
the wrong project.

app/serializers/merge_request_widget_entity.rb

@@ -139,7 +139,7 @@ class MergeRequestWidgetEntity < IssuableEntity
   expose :new_blob_path do |merge_request|
     if presenter(merge_request).can_push_to_source_branch?
-      project_new_blob_path(merge_request.project, merge_request.source_branch)
+      project_new_blob_path(merge_request.source_project, merge_request.source_branch)
     end
   end