Pre-defined push rule "Prevent committing secrets to Git" can not be enabled

Summary

When enabling the new Prevent committing secrets to Git feature

Steps to reproduce

1. On a GitLab instance running 8.12.1
2. Go to `/admin/push_rules
3. Check the box to enable Prevent committing secrets to Git
4. Click Save Push Rules

Expected behavior

The box should remain enabled, just like the other two above it.

Actual behavior

It goes back to unchecked

Relevant logs and/or screenshots

Please let me know which logs would be relevant and I will get those and add them here.