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.
If the server only supports the dumb protocol, the first 4 bytes of
response will be the initial part of the hash of the first ref.
The http-protocol documentation says that we should fall back to the
dumb protocol when we don't see a content-type of
application/x-$servicename-advertisement. Check this before
attempting to read a smart git packet.
This is implemented by checking first if the uri is
a directory containing the .git/ subdirectory.
If this is the case, we fork git/serve serving the
repository on a pipe.
