Improve ported-from update detection in the coverage script

I feel we could use the [.github/scripts/fill_prepatched_tests.sh](https://github.com/ethereum/execution-spec-tests/blob/main/.github/scripts/fill_prepatched_tests.sh#L19-L22) 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 https://github.com/ethereum/execution-spec-tests/pull/2013#discussion_r2268027391_