As I was advised on IRC:
>nix-prefetch-git https://github.com/ghcjs/ghcjs-dom
Initialized empty Git repository in /tmp/user/1000/git-checkout-tmp-uxoKqy9s/git-export/.git/
remote: Counting objects: 1070, done.
remote: Compressing objects: 100% (236/236), done.
remote: Total 1070 (delta 858), reused 932 (delta 829), pack-reused 0
Receiving objects: 100% (1070/1070), 580.67 KiB | 911.00 KiB/s, done.
Resolving deltas: 100% (858/858), done.
From https://github.com/ghcjs/ghcjs-dom
* branch HEAD -> FETCH_HEAD
Switched to a new branch 'fetchgit'
git revision is 8b9c64e78e838de95ef1b61f15c0bd7068d45d84
git human-readable version is -- none --
Commit date is 2015-06-08 03:53:22 +1200
removing `.git'...
hash is d05d04cad4aea829dddcf341ed4656d9828713d271f15c94414a74041188bac8
path is /nix/store/kcgbwampbp7qcyxqp4ag8rx2prxnsc19-git-export
d05d04cad4aea829dddcf341ed4656d9828713d271f15c94414a74041188bac8