Add a job to guard against adding changes outside of jh/

• Successful run with all changes inside jh/: https://gitlab.com/gitlab-jh/gitlab/-/jobs/1352992945
• Failed run with some changes outside of jh/: https://gitlab.com/gitlab-jh/gitlab/-/jobs/1353019685
  • Changes were a file called dummy-change: 086afb41

Closes #50 (closed)