A "old ref" of all zeroes is considered a "don't care" ref, and allows
us to say "write the new ref regardless of what the old ref contained
(or even if it existed at all)".
This allows (if git-send-pack were to do it) creating new refs, and
fixing up old ones.
After unpacking the object pack successfully, we go through the list of
refs, and verify that they still contain their expected values. Then we
replace them with the new ones.
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.
