Unexpected names and other edge cases

[weaverba137]

This PR handles a few edge cases:

• File names containing unusual unicode characters.
• Directory tree roots that are a pure symlink to another directory tree root.
• Permission errors arising from os.stat() separately from directory traversal permissions.

[coveralls]

coverage: 100.0% (+21.1%) from 78.92% when pulling 3b2e8a0 on unexpected-names into 6a6154d on main.