Don't update columns in batches in a transaction

This ensures that whatever locks are acquired aren't held onto until the
end of the transaction (= after _all_ rows have been updated). Timing
wise there's also no difference between using a transaction and not
using one.

lib/gitlab/database/migration_helpers.rb

@@ -138,11 +138,9 @@ module Gitlab
         end
 
         begin
-          transaction do
           update_column_in_batches(table, column, default, &block)
 
           change_column_null(table, column, false) unless allow_null
-          end
         # We want to rescue _all_ exceptions here, even those that don't inherit
         # from StandardError.
         rescue Exception => error # rubocop: disable all

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -90,7 +90,7 @@ describe Gitlab::Database::MigrationHelpers, lib: true do
       before do
         expect(model).to receive(:transaction_open?).and_return(false)
 
-        expect(model).to receive(:transaction).twice.and_yield
+        expect(model).to receive(:transaction).and_yield
 
         expect(model).to receive(:add_column).
           with(:projects, :foo, :integer, default: nil)