WIP: nix-prefetch-git: bring back git index (with leaveDotGit)

The git index contains timestamps. Currently we remove the index to have
deterministic output. But removing the index makes the tree "dirty". Not
good for build systems that use "git describe --dirty" to generate a
version string.

This patch changes the fetcher script to run git with libfaketime (and
time set to UNIX epoch). This way we can keep the index and still have
deterministic output.