Add support for "forcing" a ref on the remote side

Add support for "forcing" a ref on the remote side

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.