Motivation for this change

nix-prefetch-git with --quiet masks errors. This mode is consumed by scripts, and the failure is often buried several layers deep.

$ nix-prefetch-git https://github.com/sensu/yaml --rev f00f00f00; echo $?
Initialized empty Git repository in /....
[...]
fatal: 'f00f00f00' is not a commit and a branch 'fetchgit' cannot be created from it
Unable to checkout f00f00f00 from https://github.com/sensu/yaml.
1

$ nix-prefetch-git https://github.com/sensu/yaml --rev f00f00f00 --quiet; echo $?
{
  "url": "https://github.com/sensu/yaml",
  "rev": "refs/heads/fetchgit",
  "date": "",
  "sha256": "0sjjj9z1dhilhpc8pq4154czrb79z9cm044jvn75kxcjv6v5l2m5",
  "fetchSubmodules": false
}
0

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.