Improve ported-from update detection in the coverage script

[marioevz]

I feel we could use the .github/scripts/fill_prepatched_tests.sh trick where we checkout the main branch, do uv run fill $CHANGED_TEST_FILES --show-ported-from --clean --quiet --links-as-filled --skip-coverage-missed-reason --ported-from-output-file ported_from_files.txt both in main and the branch with the changes, diff the two ported_from_files.txt files, and only consider new ported tests the ones that only appear in the new branch.

Originally posted by @marioevz in #2013 (comment)