builtins.fetchgit: Support importing a working tree

For example, you can write

  src = fetchgit ./.;

and if ./. refers to an unclean working tree, that tree will be copied
to the Nix store. This removes the need for "cleanSource".

src/libexpr/primops/fetchgit.hh

@@ -17,7 +17,8 @@ struct GitInfo
 };
 
 GitInfo exportGit(ref<Store> store, const std::string & uri,
-    const std::string & ref, const std::string & rev = "",
+    std::experimental::optional<std::string> ref = {},
+    const std::string & rev = "",
     const std::string & name = "");
 
 }