Add timestamp to sources.json repositories

[Profpatsch]

nix-prefetch-git adds a timestamp to its output, which is very nice when you are skimming a sources file and want to know how old e.g. the nixpkgs version is.

nix-prefetch-git:

{
  "url": "https://github.com/nixos/nixpkgs-channels.git",
  "rev": "9beb0d1ac2ebd6063efbdc4d3631f8ce137bbf90",
  "date": "2020-01-06T05:49:56-05:00",
  "sha256": "1v95779di35qhrz70p2v27kmwm09h8pgh74i1wc72v0zp3bdrf50",
  "fetchSubmodules": false
}

niv:

    "nixpkgs": {
        "branch": "nixos-unstable",
        "description": "A read-only mirror of NixOS/nixpkgs tracking the released channels. Send issues and PRs to",
        "homepage": "https://github.com/NixOS/nixpkgs",
        "owner": "NixOS",
        "repo": "nixpkgs-channels",
        "rev": "e19054ab3cd5b7cc9a01d0efc71c8fe310541065",
        "sha256": "0b92yhkj3pq58svyrx7jp0njhaykwr29079izqn6qs638v8zvhl2",
        "type": "tarball",
        "url": "https://github.com/NixOS/nixpkgs-channels/archive/e19054ab3cd5b7cc9a01d0efc71c8fe310541065.tar.gz",
        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
    }

The same date field would be nice.

[nmattia]

@Profpatsch makes sense! This appears to be the same as #111, correct?

edit: assuming that nix-prefetch-git's timestamp is the timestamp of the latest commit

[Profpatsch]

Timestamp of latest commit makes sense, yes.

[nmattia]

Closing as duplicate of #111