The git download method clones the git repository to the local machine. The unpack process
can be optimised to be a local to local machine clone or a direct readtree operation to the
destination using git.will clone git repo to local, so git unpack can be simplified
to only checkouting the code to the work dir. For fullclone case, we also
need to manually copy all the ref info, which is needed by the later do_kernel_checkout().
Rather than use hardlinks, we reference the repository using alternatives since the
download directory may be on a different filesystem.
