DT-593: Codesniffing via pre-commit hook does not respect filesets

[danepowell]

This is the root cause of numerous other issues, most recently #3366 (which has my thoughts on the issue and some additional background), but also #2879 and others. We should fix it for real.

[danepowell]

It seems like the Twig validator handles this okay by using FilesetManager::filterFilesByFileset(), maybe replicate that pattern.

[danepowell]

To be more specific, the problem here is when you've dirtied a versioned file that isn't specifically excluded by phpcs.xml. PHPCS won't sniff files that are excluded even when specifically passed as an argument (good), but it will sniff them if they aren't already listed as a <file> (bad, if you are trying to have consistent behavior between blt validate and git commits).