Add ObjectId getByte for random access

Add ObjectId getByte for random access

Processing git notes requires random access to part of the raw data
of each ObjectId... which isn't easy because ObjectIds are stored
with an internal representation of 5 ints.  Expose random access
to the individual data bytes through new methods, avoiding the
need to convert first to a byte[20].

Change-Id: I99e64700b27fc0c95aa14ef8ad46a0e8832d4441
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>