currently ./format does a bunch of stuff with git that was only necessary for nixfmt but not nixfmt-tree, so I deleted it (just use treefmt command directly instead). I also added keep sorted to keep the really long module lists sorted (already used in nixpkgs).
- Add NIXPKGS_REV, so we can pass arbitrary revisions for testing (for
example, `release-21.05` so we can test backports).
- Add format target, that calls the format script.
The idea of this file is to make it easier to run tests. It will ensure
that the tests are running with the correct NIX_PATH (pointing it to
e.g.: unstable), and also allowing it to run either one or all tests.
