reduce delta head inflated size

reduce delta head inflated size

Supposing that both the base and result sizes were both full size 64-bit
values, their encoding would occupy only 9.2 bytes each.  Therefore
inflating 64 bytes is way overkill.  Limit it to 20 bytes instead which
should be plenty enough for a couple years to come.

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