Reclaim some bits in ObjectToPack flags field

Reclaim some bits in ObjectToPack flags field

Make the lower bits available for flags that PackWriter can use to
keep track of facts about the object.  We shouldn't need more than
2^24 delta depths, unpacking that chain is unfathomable anyway.

This change gets us 4 bits that are unused in the lower end of the
word, which are typically easier to load from Java and most machine
instruction sets.  We can use these in later changes.

Change-Id: Ib9e11221b5bca17c8a531e4ed130ba14c0e3744f
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>