nixpkgs: expose .git-revision in the nixexprs like NixOS

nixos/lib/make-channel.nix:16:

    echo -n ${nixpkgs.rev or nixpkgs.shortRev} > .git-revision

This means the .git-revision exists in nixos channels, but not
Nixpkgs channels. Adding it to the nixpkgs channel makes it a
common API for any Nixpkgs use cases.

pkgs/top-level/make-tarball.nix

@@ -24,7 +24,9 @@ releaseTools.sourceTarball rec {
     eval "$preConfigure"
     releaseName=nixpkgs-$VERSION$VERSION_SUFFIX
     echo -n $VERSION_SUFFIX > .version-suffix
+    echo -n ${nixpkgs.rev or nixpkgs.shortRev} > .git-revision
     echo "release name is $releaseName"
+    echo "git-revision is $(cat .git-revision)"
   '';
 
   dontBuild = false;