One pack in a Gitility.PackManifest: storage keys for the pack and its
index, sizes for planning fetches, and the pack checksum as identity.
Summary
Types
@type t() :: %Gitility.PackDescriptor{ etag: binary() | nil, id: binary(), index_key: binary(), index_size: non_neg_integer(), pack_key: binary(), pack_size: non_neg_integer() }