diff: Improve directory loop detection

[des]

Summary

When we're done processing a directory, remove its entry from the tree
of visited inodes, ensuring that we only report a loop when we encounter
a descendant-to-ancestor link, not when we encounter a cousin-to-cousin
or sibling-to-sibling link.

MFC after: 1 week
Sponsored by: Klara, Inc.

[des]

move null check