ReceivePack: Discard IndexPack as soon as possible

The IndexPack object carries a good bit of state within itself about
the objects received over the wire.  The earlier we can discard it,
the sooner the GC is able to reclaim this chunk of memory for other
uses.  So drop it as soon as we are certain the pack is valid and we
have no connectivity concerns.

Change-Id: I1e8bc87c2e9183733043622237a064e55957891f
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>