`git-annex find --not --unused` is currently the same as `git-annex find
--unused` because `--unused` is not a file matching option. This is a bit
confusing. 

And it would be useful to have a way to find all keys
that are not amoung the unused keys.

Given the implementation of `--not` is tied to file matching options,
it might be best to add a new option like `--used` or `--not-unused`.

It would also perhaps be good to detect when matching options are used that
don't make sense, and error out on commands like `git-annex find --not`
or `git-annex find -and -(`
