Accept units (k, m, g) for --chunk-size.

Add option -k/--keep-going to continue in case of IO errors such as permission denied.

Add option -e/--errexit to immediately exit >0 on the first denied difference.

Add option to scan ahead and show progress on standard error.

Hash and check symlinks targets?
  https://doc.rust-lang.org/std/fs/fn.read_link.html
  https://unix.stackexchange.com/questions/63876/changing-the-timestamp-of-a-symlink

Estimate default number of worker threads based on disk read speed and CPU speed.

Test effect of stdout buffering and/or locking on performance with high verbosity.
  https://rust-cli.github.io/book/tutorial/output.html#a-note-on-printing-performance

Generate a man page.
  https://rust-cli.github.io/book/in-depth/docs.html
  https://lib.rs/crates/clap_mangen

Use common exit codes.
  https://man.openbsd.org/sysexits
  https://github.com/dtolnay/anyhow/issues/247