Git9 was sloppy about telling git what commits we have.
We would list the commits at the tip of the branch, but not
walk down it, which means we would request too much data if
our local branches were ahead of the remote.
This patch changes that, sending the tips *and* the first
256 commits after them, so that git can produce a better
pack for us, with fewer redundant commits.
bb33663b40