ee-specific-lines-check, Show diff because git status might not be readable

What does this MR do?

Show diff because git status might not be readable: https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/82553270

Can you spot this?

💥 Git status not clean! This means there's a conflict in
💥 target-ce/ce-5158-metrics-alerting with canonical-ce/master. Please resolve
💥 the conflict from CE master and retry this job.

If we see diff with conflicts we could immediately guess what happened.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

Similar to: https://gitlab.com/gitlab-org/gitlab-ee/issues/6856