This has the disadvantage that we can only use the current tree to
collect evidence of overlap -- this means that it's possible for file
addition or deletion to force a CODEOWNERS update.

HOWEVER, it has the advantage that it's correct and propagates ownership
in the way that we want (alleviating murgatroid@ from having to approve
every additional file in the repository)

-) Paths should all be absolute to /
-) Computing the intersection of two globs should happen with the
full path in there, not just the directory's glob.