v0.4.0

Breaking change: exit codes are inverted. 0 now means "no duplicates"
(the clean, nothing-to-act-on state) and 1 means "duplicates were
found". 2 for errors is unchanged. This matches the linter convention
used by tools like clippy and shellcheck, and avoids set -e
scripts tripping on a successful scan that simply found nothing.

If you scripted the previous behaviour, swap the branches:
fifi /backup && handle_dupes becomes fifi /backup || handle_dupes.

See CHANGELOG.md for the full entry.