fetch: default to protocol version 2

fetch: default to protocol version 2

The Git users at $DAYJOB have been using protocol v2 as a default for
~1.5 years now and others have been also reporting good experiences
with it, so it seems like a good time to bump the default version.  It
produces a significant performance improvement when fetching from
repositories with many refs, such as
https://chromium.googlesource.com/chromium/src.

This only affects the client, not the server.  (The server already
defaults to supporting protocol v2.)  The protocol change is backward
compatible, so this should produce no significant effect when
contacting servers that only speak protocol v0.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>