Uh, where is it shown how "software quality" or "code quality" is measured or determined? Can anyone provide a succinct definition of quality which the paper uses?

As best I can tell, they use commit messages to identify bugfixes, and later they jump to "defective commits". Presumably the bugfix commit is not the defective commit. There is no explanation I can find that shows how they arrive at a defective commit from a bugfix commit.

This specific methodology seems rife with weakness, all of which should be explained clearly and admitted up front.