fetch-pack: factor out hardcoded handshake window size

fetch-pack: factor out hardcoded handshake window size

The "git fetch" client presents the most recent 32 commits it has to the
server and gives a chance to the server to say "ok, we heard enough", and
continues reporting what it has in chunks of 32 commits, digging its
history down to older commits.

Move the hardcoded size of the handshake window outside the code, so that
we can tweak it more easily.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Shawn Pearce <spearce@spearce.org>