ReceivePack: Remove need new,base object id properties

These are more like internal implementation details of how IndexPack
works with ReceivePack to validate the incoming object stream.
Callers who are embedding the ReceivePack logic in their own
application don't really need to know the details of which objects
were used for delta bases in the incoming thin pack, or exactly
which objects were newly transmitted.

Hide these from the API, as exposing them through ReceivePack was
an early mistake.

Change-Id: I7ee44a314fa19e6a8520472ce05de92c324ad43e
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>