Spec failure in `spec/controllers/projects/merge_requests/diffs_controller_spec.rb` | Projects::MergeRequests::DiffsController GET show with default params for the same project serializes merge request diff collection

Job #1514915899 failed for 6b7ded28:

Trace

Failures:
  1) Projects::MergeRequests::DiffsController GET show with default params for the same project serializes merge request diff collection
     Failure/Error:
       target.to receive_new.and_wrap_original do |method, *original_args|
         method.call(*original_args).tap(&blk)
       end
       (DiffsSerializer (class)).new(*(any args))
           expected: 1 time with any arguments
           received: 0 times with any arguments
     # ./spec/support/helpers/next_instance_of.rb:28:in `stub_new'
     # ./spec/spec_helper.rb:390:in `block (3 levels) in <top (required)>'
     # ./spec/support/sidekiq_middleware.rb:9:in `with_sidekiq_server_middleware'
     # ./spec/spec_helper.rb:381:in `block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:377:in `block (3 levels) in <top (required)>'
     # ./lib/gitlab/application_context.rb:31:in `with_raw_context'
     # ./spec/spec_helper.rb:377:in `block (2 levels) in <top (required)>'