Update syntax highlighting logic [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Contributes to #16950 (closed)

We apply syntax highlighting to old and new versions of the whole file. However, we only need to highlight requested diff lines.

This change should optimize this process. Here we provide only necessary diff lines to the syntax highlighter.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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