Add first cut at "git-receive-pack"

Add first cut at "git-receive-pack"

It's not working yet, but it's at the point where I want to be able to
track my changes.  The theory of operation is that this is the "remote"
side of a "git push".  It can tell us what references the remote side
has, receives out reference update commands and a pack-file, and can
execute the unpacking command.