Gitlab::Mirror spec fails at certain times of the day

If you run bundle exec spec/lib/gitlab/mirror_spec.rb https://gitlab.com/gitlab-org/gitlab-ee/builds/13669226 sometime between 00:00 and 01:00 UTC, the spec fails because:

DateTime.now.beginning_of_hour == DateTime.now.at_beginning_of_day

There may be other instances where this occurs too.

/cc: @tiagonbotelho