t1006: rename sha1 to oid

t1006: rename sha1 to oid

Before I extend this test, changing the naming of the relevant
hash from sha1 to oid.  Calling the hash sha1 is incorrect today
as it can be either sha1 or sha256 depending on the value of
GIT_DEFAULT_HASH_FUNCTION when the test is called.

I plan to test sha1 and sha256 simultaneously in the same repository.
Having a name like sha1 will be even more confusing.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>