fetchgit: add 'deepClone' argument to disable shallow fetching

This patch resolves https://github.com/NixOS/nixpkgs/issues/6395. Deep
cloning is useful in combination with 'leaveDotGit' for builds that want
to run "git describe" to obtain a proper version string, etc., like the
'haskellngPackages.cabal2nix' package does.