Show the conflicted diff only in ee-specific-lines-check

What does this MR do?

Show the conflicted diff only in ee-specific-lines-check

Why was this MR needed?

Look at https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/80330885

Those modifications were actually clean. The only important information was:

UU app/models/ci/build_trace_chunk.rb

We could just print the diff contents so it's much easier to diagnose.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

Closes #6856 (closed)