The update_all method doesn't return the number of rows affected when combining the relation with in_batches.

[khiav223577]

Steps to reproduce

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"
  gem "activerecord", '~> 6.0.3'
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table "users", force: :cascade do |t|
  end
end

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
end

class User < ApplicationRecord
end

User.transaction do
  3.times do
    User.create
  end
end

class BugTest < Minitest::Test
  def test_update_all
    assert_equal 3, User.update_all('id = id')
  end

  def test_update_all_in_batches
    assert_equal 3, User.in_batches.update_all('id = id')
  end
end

=begin
Result:

  1) Failure:
BugTest#test_update_all_in_batches [test.rb:45]:
Expected: 3
  Actual: nil
=end

Expected behavior

Expect the update_all method always returns the number of rows affected.

Actual behavior

The return value of in_batches.update_all is nil.

System configuration

Rails version:
Rails 6.0.3.2

Ruby version:
ruby 2.6.4p104 (2019-08-28 revision 67798) [x86_64-linux]