obadz 47950b5353 modules/misc/version.nix: populate nixosRevision based on <nixpkgs/.git> when possible (#15624)
Example:

$ nixos-option system.nixosLabel
Value:
"16.09.git.4643ca1"
2016-05-24 23:34:28 +01:00
..
2016-05-05 21:28:27 +01:00
2016-05-24 10:49:03 +02:00