pack-object: slightly more efficient

pack-object: slightly more efficient

Avoid creating a delta index for objects with maximum depth since they
are not going to be used as delta base anyway.  This also reduce peak
memory usage slightly as the current object's delta index is not useful
until the next object in the loop is considered for deltification. This
saves a bit more than 1% on CPU usage.

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