get rid of num_packed_objects()

get rid of num_packed_objects()

The coming index format change doesn't allow for the number of objects
to be determined from the size of the index file directly.  Instead, Let's
initialize a field in the packed_git structure with the object count when
the index is validated since the count is always known at that point.

While at it let's reorder some struct packed_git fields to avoid padding
due to needed 64-bit alignment for some of them.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>