Detect symlink loops

[israel-lugo]

Now that we have the --follow-dirlinks option (see #4), we need a way to detect symlink loops. If there is a symlink pointing to ., or to a parent directory, we will go in a loop.

Fortunately, os.walk() seems to stop after several levels of recursion. But still, it's probably undefined behavior.

See what commands like find or rsync do.

[israel-lugo]

This must actually be fixed in the underlying capidup library. See israel-lugo/capidup#17. We still need to devise some way to report to the user when a loop has been detected; that is a UI concern, appropriate for capidup-cli.