pack-objects: do not stop at object that is "too small"

pack-objects: do not stop at object that is "too small"

Because we sort the delta window by name-hash and then size,
hitting an object that is too small to consider as a delta base
for the current object does not mean we do not have better
candidate in the window beyond it.

Noticed by Shawn Pearce, analyzed by Nico, Linus and me.

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