Initially the conversion was made using nth_packed_object_sha1() which
made this file completely index version agnostic. Unfortunately the
overhead was quite significant so I went back to raw index walking but
with selectable base and step values which brought back similar
performances as the original.
Signed-off-by: Nicolas Pitre <nico@cam.org> Signed-off-by: Junio C Hamano <junkio@cox.net>