Revert "move pack creation to version 3"

Revert "move pack creation to version 3"

This reverts commit 16854571aae6302f457c5fbee41ac64669b09595.
Git as recent as v1.1.6 do not understand version 3 delta.

v1.2.0 is Ok and I personally would say it is old enough, but
the improvement between version 2 and version 3 delta is not
bit enough to justify breaking older clients.

We should resurrect this later, but when we do so we shold
make it conditional.

Signed-off-by: Junio C Hamano <junkio@cox.net>