fetchFromGitHub: Revert to the original version

fetchFromGitHub was intended as a simple wrapper around the common
pattern 'fetchzip { url =
https://github.com/${owner}/${repo}/archive/${rev}.zip"; ... }'. It
was not intended to handle private repositories, submodules, other
sites than github.com (!), etc. In particular, we don't want to use
fetchgit because it's not reproducible.