Creating deltas between big blobs is a CPU and memory intensive task.
In the writing phase, all (not reused) deltas are redone.

This patch adds support for caching deltas from the deltifing phase, so
that that the writing phase is faster.

The caching is limited to small deltas to avoid increasing memory usage very much.
The implemented limit is (memory needed to create the delta)/1024.
Signed-off-by: NMartin Koegler <mkoegler@auto.tuwien.ac.at>
Signed-off-by: NJunio C Hamano <junkio@cox.net>