Ignore gitignored files in tidy's file counting

[thomcc]

Recently I apparently ended up with a .DS_Store file inside src/test/ui. This caused ./x.py test tidy to complain about:

tidy error: following path contains more than 968 entries, you should move the test to some relevant subdirectory (current: 969): /Users/thom/src/rust/src/test/ui

This would be reasonable, however I have these files globally gitignored. @jyn514 suggested that maybe it would be good if tidy didn't complain about gitignored files, and asked me to file an issue. This is that issue.

Forgive the imprecise title, I have no idea what this tidy test is trying to verify (and probably wouldn't file this issue were it not for the request -- It's not unreasonable for me to require stuff like .DS_Store not be present). I also don't know what the right label for tidy is.

[jyn514]

cc #98793 (review), we've been meaning to do this for a while