Add more debug to 'auto_deploy:prepare'
It came up in a comment https://gitlab.com/gitlab-org/gitlab-ce/issues/66479#note_207091019 where a developer thought a change would be in the auto-deploy branch but it did not make it. In addition https://gitlab.com/gitlab-org/release/framework/issues/368 maybe we should add some extra debug to this job output so it will also display the date where we found the latest green commit https://gitlab.com/gitlab-org/release-tools/blob/master/lib/release_tools/services/auto_deploy_branch_service.rb#L23-27