compare to last commit in a PR when using git commands

[mirpedrol]

Close #205
Follow-up from #133

[dorny]

@mirpedrol The #133 is merged now. Does it solve also your issue?
Fix in your PR might be working for you, but modifying getCurrentRef() to use HEAD^2 doesn't look correct. Although I haven't tried it myself.

[mirpedrol]

Thanks for merging #133!
The fix in #133 allowed us to use this action without using the API, but we still have the problem of detecting too many files changed.
This happens because the last commit on PR events is the pre-merge commit, so it includes files that are not modifyed by the current PR.
Would there be a better way to prevent that without having to use HEAD^2?