* Rework change detection via `git diff`
Previous implementation performed simple diff between two versions. New implementation fetches on demand more commits to have the merge base between two branches. Now it will detect only changes introduced by branch that was pushed, instead of mixing with changes introduced meanwhile on the base branch.
* Add support for specification of change type (add,modified,delete)
* Use NULL as separator in git-diff command output
* Improve PR test workflow
* Fix the workflow file
