use delta index data when finding best delta matches

use delta index data when finding best delta matches

This patch allows for computing the delta index for each base object
only once and reuse it when trying to find the best delta match.

This should set the mark and pave the way for possibly better delta
generator algorithms.

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