index-pack: smarter memory usage during delta resolution

index-pack: smarter memory usage during delta resolution

There is no need to keep the base object data around after its last delta
has been resolved.  This also means that long delta chains with only one
delta per base won't grow the cache size unnecessarily as the base will
be freed before recursing down.

To make it easy, find_delta_children() is modified so the first and last
indices are initialized in all cases.

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