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

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

By running git under libfaketime (with time set to UNIX epoch) we don't
have to delete the index and can still have deterministic output.
libfaketime: new package

Report faked system time to programs without having to change the
system-wide time.

http://www.code-wizards.com/projects/libfaketime/