Commit 06aaaa0bf70fe37d198893f4e25fa73b6516f8a9 may step index format
version up and down, depends on whether extended flags present in the
index. This adds a test to check for index format version.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
