Add 'nix diff-closures' command

This command makes it easier to see what changed between two closures,
i.e. what packages/versions got added or removed, and whether there
were any notable changes in path size.

For example:

  $ nix diff-closures /nix/var/nix/profiles/system-655-link /nix/var/nix/profiles/system-658-link
  blender-bin: 2.83.0 → 2.83.2, -294.2 KiB
  curl: 7.68.0 → 7.70.0, +19.1 KiB
  firmware-linux-nonfree: 2020-01-22 → 2020-05-19, +30827.7 KiB
  ibus: -21.8 KiB
  initrd-linux: 5.4.46 → 5.4.51, +16.9 KiB
  libexif: 0.6.21 → 0.6.22, +497.6 KiB
  linux: 5.4.46 → 5.4.51, +13.2 KiB
  mesa: 19.3.3 → 19.3.5, -183.9 KiB
  nix: 2.4pre20200701_6ff9aa8 → 2.4pre20200708_9223603, +9.7 KiB
  nix-bash-completions: 0.6.8 → ∅, -57.6 KiB
  nixos-system-hagbard: 20.03.20200615.a84b797 → 20.03.20200713.add5529
  nvidia-persistenced: 440.82 → 440.100
  nvidia-settings: 440.82 → 440.100
  nvidia-x11: 440.82-5.4.46 → 440.100-5.4.51, +664.7 KiB
  pcre: 8.43 → 8.44
  php: 7.3.16 → 7.3.20, -26.2 KiB
  python3.7-youtube-dl: 2020.06.06 → 2020.06.16.1, +8.4 KiB
  samba: 4.11.5 → 4.11.9, +30.1 KiB
  sane-backends: 1.0.28 → 1.0.30, +680.5 KiB
  source: -182.0 KiB
  zfs-kernel: 0.8.3-5.4.46 → 0.8.4-5.4.51, +9.9 KiB
  zfs-user: 0.8.3 → 0.8.4, +20.1 KiB

0 files changed, 0 insertions, 0 deletions