Research: implement subtract on existing merge/join

[dmpetrov]

We need to research these areas:

• subtract() uses custom machinery and runs sql-join under the hood. Can we level this up to application level and use existing datachain merge/join?

• user might need several outputs like git diff - added, removed, changed. We need a diff command that supports all of these like ch1.diff(ch2, status="ad") (added and deleted files only).

The result of this issue should be a proposal for (1) and (2), not implementation.