diff-delta.c: pack the index structure

diff-delta.c: pack the index structure

In normal use cases, the performance wins are not overly impressive:
we get something like 5-10% due to the slightly better locality of
memory accesses using the packed structure.

However, since the data structure for index entries saves 33% of
memory on 32-bit platforms and 40% on 64-bit platforms, the behavior
when memory gets limited should be nicer.

This is a rather well-contained change.  One obvious improvement would
be sorting the elements in one bucket according to their hash, then
using binary probing to find the elements with the right hash value.

As it stands, the output should be strictly the same as previously
unless one uses the option for limiting the amount of used memory, in
which case the created packs might be better.

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