t6500: specify test values for SHA-256

t6500: specify test values for SHA-256

In this test, we want to produce several blobs whose first two hex
characters are "17", since we look at this object directory as a proxy
for how many loose objects there are before we need to GC.  Use
test_oid_cache to specify strings that will hash to the right values
when turned into blobs.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>