Edge case: overlapping roots with --ignore

[defuse]

Claude said: "If one root is a prefix of the other (e.g. original=/data, backup=/data/backup) AND the backup path involves symlinks, the four-branch strip_prefix chain in cli.rs:229-245 can match an ignore path against the wrong root. For example, --ignore /data/backup/cache would strip the /data original prefix first (getting suffix backup/cache) and store canonical_original/backup/cache, which wouldn't match the walked path canonical_backup/cache if /data/backup resolves differently through canonicalization."

It might be fine to leave it as-is, but at least document it in the comment in the code.