An object's type and size without its payload.
Headers answer "what is this and how big" cheaply — for packed objects
even the size may require some work, which is why tree listings omit blob
sizes unless asked (include: [:size]).
Summary
Types
@type t() :: %Gitility.ObjectHeader{ oid: Gitility.OID.t(), size: non_neg_integer(), type: Gitility.Object.object_type() }