[Rails5] Fix Arel::UpdateManager

In Arel 7.0.0 (Arel 7.1.4 is used in Rails 5.0) the `engine` parameter
of `Arel::UpdateManager#initializer` was removed.

This commit makes the gitlab database helpers work both in rails 4 and
rails 5.
Fixes errors like this one:

```
1) Gitlab::Database::MigrationHelpers#update_column_in_batches when running outside of a transaction updates all the rows in a table
    Failure/Error:
      update_arel = Arel::UpdateManager.new(ActiveRecord::Base)
        .table(table)
        .set([[table[column], value]])
        .where(table[:id].gteq(start_id))

    ArgumentError:
      wrong number of arguments (given 1, expected 0)
    # ./lib/gitlab/database/migration_helpers.rb:317:in `new'
    # ./lib/gitlab/database/migration_helpers.rb:317:in `block in update_column_in_batches'
    # ./lib/gitlab/database/migration_helpers.rb:307:in `loop'
    # ./lib/gitlab/database/migration_helpers.rb:307:in `update_column_in_batches'
    # ./spec/lib/gitlab/database/migration_helpers_spec.rb:367:in `block (4 levels) in <top (required)>'
```

lib/gitlab/database/arel_methods.rb

+module Gitlab
+  module Database
+    module ArelMethods
+      private
+
+      # In Arel 7.0.0 (Arel 7.1.4 is used in Rails 5.0) the `engine` parameter of `Arel::UpdateManager#initializer`
+      # was removed.
+      # Remove this file and inline this method when removing rails5? code.
+      def arel_update_manager
+        if Gitlab.rails5?
+          Arel::UpdateManager.new
+        else
+          Arel::UpdateManager.new(ActiveRecord::Base)
+        end
+      end
+    end
+  end
+end

lib/gitlab/database/migration_helpers.rb

 module Gitlab
   module Database
     module MigrationHelpers
+      include Gitlab::Database::ArelMethods
+
       BACKGROUND_MIGRATION_BATCH_SIZE = 1000 # Number of rows to process per job
       BACKGROUND_MIGRATION_JOB_BUFFER_SIZE = 1000 # Number of jobs to bulk queue at a time
 
@@ -314,7 +316,7 @@ module Gitlab
           stop_arel = yield table, stop_arel if block_given?
           stop_row = exec_query(stop_arel.to_sql).to_hash.first
 
-          update_arel = Arel::UpdateManager.new(ActiveRecord::Base)
+          update_arel = arel_update_manager
             .table(table)
             .set([[table[column], value]])
             .where(table[:id].gteq(start_id))

lib/gitlab/database/rename_reserved_paths_migration/v1/rename_base.rb

@@ -3,6 +3,8 @@ module Gitlab
     module RenameReservedPathsMigration
       module V1
         class RenameBase
+          include Gitlab::Database::ArelMethods
+
           attr_reader :paths, :migration
 
           delegate :update_column_in_batches,
@@ -62,7 +64,7 @@ module Gitlab
                                             old_full_path,
                                             new_full_path)
 
-            update = Arel::UpdateManager.new(ActiveRecord::Base)
+            update = arel_update_manager
               .table(routes)
               .set([[routes[:path], replace_statement]])
               .where(Arel::Nodes::SqlLiteral.new(filter))