builtins.fetchgit: Support specifying commit hashes

This adds an argument "rev" specififying the Git commit hash. The
existing argument "rev" is renamed to "ref". The default value for
"ref" is "master". When specifying a hash, it's necessary to specify a
ref since we're not cloning the entire repository but only fetching a
specific ref.

Example usage:

  builtins.fetchgit {
    url = https://github.com/NixOS/nixpkgs.git;
    ref = "release-16.03";
    rev = "c1c0484041ab6f9c6858c8ade80a8477c9ae4442";
  };

src/libexpr/primops/fetchgit.hh

@@ -8,7 +8,7 @@ namespace nix {
 
 class Store;
 
-Path exportGit(ref<Store> store,
-    const std::string & uri, const std::string & rev);
+Path exportGit(ref<Store> store, const std::string & uri,
+    const std::string & ref, const std::string & rev = "");
 
 }