Reify policy for monorepos

## Problem statement

https://github.com/nedap/formatting-stack/pull/137#issuecomment-593383693 mentions a policy for how to deal with monorepos.

* That's followed in `git ls-files`, but not for `status` or `diff`
* There are no tests around that

## Proposal

Refine and reify policy

## Alternatives and comparison

?
