packed_object_info: hoist delta type resolution to helper

packed_object_info: hoist delta type resolution to helper

To calculate the type of a packed object, we must walk down
its delta chain until we hit a true base object with a real
type. Most of the code in packed_object_info is for handling
this case.

Let's hoist it out into a separate helper function, which
will make it easier to make the type-lookup optional in the
future (and keep our indentation level sane).

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>