Also consider that if its checked on a commit by commit basis that doesn't collapse merges in somehow, then you actually check every commit that was made during

Also consider that if its checked on a commit by commit basis that doesn't collapse merges in somehow, then you actually check every commit that was made during incremental work on a different branch. If you're doing TDD or any test first methodology, that's going to often mean you intentionally commit broken tests to fix them. Also if its a commit by commit basis, the percentage of time comparison is completely invalid.