make the pack index version configurable

make the pack index version configurable

It is a good idea to use pack index version 2 all the time since it has
proper protection against propagation of certain pack corruptions when
repacking which is not possible with index version 1, as demonstrated
in test t5302.

Hence this config option.

The default is still pack index version 1.

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