--copyfrom gets option to enforce copying NULL attributes to the destination

[porg]

Status Quo

-f, --copyfrom SOURCE_FILE
Copy attributes from file SOURCE_FILE (only updates destination attributes that are not null in SOURCE_FILE).

Proposal

• When running --copyfrom with --mirror-null then NULL values on the SRC are nulled on the DST side too.

Design Rationale

• This is handy when using osxmetadata --copyfrom /local/file /remote/file as a means of metadata sync to backup destinations which are ok for backing up your data but may loose metadata sometimes, hence the need so sync from time to time.
• In situations where a Finder tag or comment gets purposely changed from e.g. "Current Marketing Material" to "", it would be nice if I could push those intentional removal with an enforcing flag like --mirror-null.

[porg]

Maybe the --all option was made in that spirit?

• add --all option to backup and restore #33 has been worked on but is open, and it's last comment says:

Also add for --mirror and --copyfrom

• Which possibly is the same intention as this ticket. If so feel free to close/merge as you see fit.