Fix Ruby 2.7 warnings in file collection specs

In Ruby 3.0, positional arguments and keyword arguments will be
separated. In Ruby 2.7, deprecation warnings related to keyword
arguments show up.

Use a double splat to specify `collection_default_args` as keyword
arguments. The `diff statistics` shared example is also used in
other file collection specs so the warnings in those specs are
also fixed.

spec/lib/gitlab/diff/file_collection/merge_request_diff_batch_spec.rb

@@ -120,7 +120,7 @@ RSpec.describe Gitlab::Diff::FileCollection::MergeRequestDiffBatch do
       described_class.new(merge_request.merge_request_diff,
                           batch_page,
                           batch_size,
-                          collection_default_args)
+                          **collection_default_args)
     end
   end
 

spec/support/shared_examples/lib/gitlab/diff_file_collections_shared_examples.rb

 # frozen_string_literal: true
 
 RSpec.shared_examples 'diff statistics' do |test_include_stats_flag: true|
-  subject { described_class.new(diffable, collection_default_args) }
+  subject { described_class.new(diffable, **collection_default_args) }
 
   def stub_stats_find_by_path(path, stats_mock)
     expect_next_instance_of(Gitlab::Git::DiffStatsCollection) do |collection|