Fix N+1 with source projects in merge requests API

Now that we are checking `MergeRequest#for_fork?`, we also need
the source project preloaded for a merge request.

changelogs/unreleased/sh-fix-source-project-nplus-one.yml

+---
+title: Fix N+1 with source_projects in merge requests API
+merge_request:
+author:
+type: performance

lib/api/merge_requests.rb

@@ -38,7 +38,7 @@ module API
         merge_requests = MergeRequestsFinder.new(current_user, args).execute
                            .reorder(args[:order_by] => args[:sort])
         merge_requests = paginate(merge_requests)
-                           .preload(:target_project)
+                           .preload(:source_project, :target_project)
 
         return merge_requests if args[:view] == 'simple'