bisect: make skipped array functions more generic

bisect: make skipped array functions more generic

So they can be used on the good array too.

This is done by renaming many functions and some variables to
remove "skip" in the name, and by adding a
"struct sha1_array *array" argument where needed.

While at it, make the second argument to "lookup_sha1_array"
const. It becomes "const unsigned char *sha1".

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>