t6500: extract objects with "17" prefix

t6500: extract objects with "17" prefix

The ".git/obects/17/" shard is somewhat special because it is used by
git-gc(1) to estimate how many objects there are by extrapolating the
number of objects in that shard, only. In t6500 we thus have a hard
coded set of data that, when written to the object database, result in
blobs starting with that prefix.

We are about to need such "17"-prefixed objects in another test suite.
Extract them into "t/oid-info/hash-info" so that they can be reused by
other tests.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>