Add a utility function "fetchGitHub"

This is a small wrapper around fetchzip. It allows you to say:

  src = fetchGitHub {
    owner = "NixOS";
    repo = "nix";
    rev = "924e19341a5ee488634bc9ce1ea9758ac496afc3"; # or a tag
    sha256 = "1ld1jc26wy0smkg63chvdzsppfw6zy1ykf3mmc50hkx397wcbl09";
  };

pkgs/tools/networking/dd-agent/default.nix

@@ -1,12 +1,14 @@
-{ stdenv, fetchzip, python, pythonPackages, sysstat, unzip, tornado
+{ stdenv, fetchGitHub, python, pythonPackages, sysstat, unzip, tornado
 , makeWrapper }:
 
 stdenv.mkDerivation rec {
   version = "4.2.1";
   name = "dd-agent-${version}";
 
-  src = fetchzip {
-    url = "https://github.com/DataDog/dd-agent/archive/${version}.zip";
+  src = fetchGitHub {
+    owner = "DataDog";
+    repo = "dd-agent";
+    rev = version;
     sha256 = "06f9nkvnpfzs2nw75cac2y9wnp2bay4sg94zz0wjm8886rigjjjm";
   };