Invalid repository compare causes 500

Production was alerted because of a sharp increase in 500s due to a user doing penetration testing and hitting endpoints that caused a huge increase in 500 errors.

Example: https://gitlab.com/bretbrownjr/community-study/compare/b2153ec857eb5003cdc7dcdce71f25928deb0b00...691%24%7B98991*97996%7D

https://log.gitlab.net/goto/9185158036f3848d46b6fa825b815793

this error should be handled in a way that doesn't blow up and return a 500.