Dont highlight untouched lines when diffing diffs
## Description When reviewing MR's that change diffs, it's already confusing to tell what's changed vs. what's not. Let's emphasize what's changed by only highlighting the actual changed lines in a diff. ![Screenshot_2024-08-15_at_11.12.18_AM](/uploads/085f3f7d1cc237173e541b6e28cd2f2e/Screenshot_2024-08-15_at_11.12.18_AM.png) <small>Taken from this MR https://gitlab.com/gitlab-org/gitlab/-/merge_requests/162889</small>
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