Problem: nix-prefetch-git output is noisy and confusing

[clacke]

The user might not believe that this much output is the result of a successful call to racket2nix.

Solution: Swallow the stderr, maybe providing a brief progress indicator like "Fetching git://blah/...". Only reveal the full stderr if nix-prefetch-git fails.

Initialized empty Git repository in /private/var/folders/4h/667g12ds42s7tnt7rwqyk0hw0000gn/T/git-checkout-tmp-vw4vAApW/typed-map-c9c5a23/.git/
remote: Counting objects: 25, done.
remote: Compressing objects: 100% (20/20), done.
remote: Total 25 (delta 3), reused 14 (delta 0), pack-reused 0
Unpacking objects: 100% (25/25), done.
From git://github.com/jsmaniac/typed-map
 * branch            HEAD       -> FETCH_HEAD
Switched to a new branch 'fetchgit'
removing `.git'...

git revision is c9c5a236f4e32d9391df3edffdf9b1a55401fe31
path is /nix/store/iqwr50i4553bnpmy2a7m7j4ginivap42-typed-map-c9c5a23
git human-readable version is -- none --
Commit date is 2018-01-27 13:25:01 +0100
hash is 150agc51y1kvrarg0n6r2x6n3awnvivqj5k78gx9ngr8q31zl83f

[sjmackenzie]

yes this confused me