fetch: never unpack git fetch packs

fetch: never unpack git fetch packs

While not necessary to avoid a race condition, it's much easier to
manage incoming fetch data by keeping it in packs and periodically
joining them together as needed.

This also can result in a minor space savings since pack storage is
typically more efficient than loose object storage.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>