packfile: disentangle return value of `packed_object_info()`

packfile: disentangle return value of `packed_object_info()`

The `packed_object_info()` function returns the type of the packed
object. While we use an `enum object_type` to store the return value,
this type is not to be confused with the actual object type. It _may_
contain the object type, but it may just as well encode that the given
packed object is stored as a delta.

We have removed the only caller that relied on this returned object type
in the preceding commit, so let's simplify semantics and return either 0
on success or a negative error code otherwise.

This unblocks a small optimization where we can skip reading the object
type altogether.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>