Comparing against the wrong base (master) commit

[EnricoMi]

The action compares a commit of a branch to the where it branched off master. Github seems to merge this commit with master head, so unit tests include master head and should compared against that (at least in pull_request_target). This has happened here. Make sure to compare to the master commit that is part of the merge, not master head, as master could have moved further while the action is running.

What if that commit cannot be merged into master? On which commit does GitHub actions run then?

[EnricoMi]

This has happened there because it ran on pull_request_target event, which points to master head. That way we always build and test master head. This is a duplicate of #45.