pack.indexversion config option now defaults to 2

pack.indexversion config option now defaults to 2

As announced for 1.6.0.

Git older than version 1.5.2 (or any other git version with this option
set to 1) may revert to version 1 of the pack index by manually deleting
all .idx files and recreating them using 'git index-pack'.  Communication
over the git native protocol is unaffected since the pack index is never
transferred.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>