grep: grep cache entries if they are "assume unchanged"

grep: grep cache entries if they are "assume unchanged"

"Assume unchanged" bit means "please pretend that I have never touched
this file", so  if user removes the file, we should not care.

This patch teaches "git grep" to use cache version in such
situations. External grep case has not been fixed yet. But given that
on the platform that CE_VALID bit may be used like Windows, external
grep is not available anyway, I would wait for people to raise their
hands before touching it.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>