Create annotations based on diff-coverage, not full coverage

[ewjoachim]

This is a revamp of #188

I believe that the part that failed was:
https://github.com/py-cov-action/python-coverage-comment-action/pull/188/files#diff-5327f4a45499e33cab8758f61e6b738fc200a07b00c58fc231f37a3142461ac7R194-R207

which I removed completely from this PR, as it wasn't needed.

1/ This PR being created by me, we'll get e2e tests
2/ I've remade the commits and remove the things are are much unrelated. We still have a 1st commit that could be somewhat elsewhere, but we were being really inconsistent in types used, and it was visible when switching between cover and diff-cover, so hopefully it's better now

[github-actions]

End-to-end public repo
End-to-end private repo
If the tests fail, @ewjoachim will be added to the private repo

[github-actions]

Coverage report

The coverage rate went from 100% to 100% ➡️
The branch rate is 100%.

100% of new lines are covered.

Diff Coverage details (click to unfold)

coverage_comment/annotations.py

100% of new lines are covered (100% of the complete file).

coverage_comment/coverage.py

100% of new lines are covered (100% of the complete file).

coverage_comment/main.py

100% of new lines are covered (100% of the complete file).

coverage_comment/github.py

100% of new lines are covered (100% of the complete file).

[kieferro]

Thanks, that looks great