We only cache deltas when it's smaller than a certain limit. This limit
defaults to 1000 but save its compressed length in a 64-bit field.
Shrink that field down to 20 bits, so you can only cache 1MB deltas.
Larger deltas must be recomputed at when the pack is written down.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>