Implement multi_ack_detailed protocol extension

The multi_ack_detailed extension breaks out the "ACK %s continue" status
code into "ACK %s common" and "ACK %s ready" states, making it easier to
discover which objects are truely common, and which objects are simply
on a chain the server doesn't care learning about.

Change-Id: Ie8e907424cfbbba84996ca205d49eacf339f9d04
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>