This is not used anywhere yet. But the goal is to compare quickly if a
.gitignore file has changed when we have the SHA-1 of both old (cached
somewhere) and new (from index or a tree) versions.
Helped-by: Junio C Hamano <gitster@pobox.com> Helped-by: Torsten Bögershausen <tboegi@web.de> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>