t5516: prefer "oid" to "sha1" in some test titles

t5516: prefer "oid" to "sha1" in some test titles

These old tests refer to object ids as "sha1". These days we prefer
the more algorithm-agnostic "oid".

There are a few more tests that mention sha1 in the title and also use
it in variables throughout the test. I've left them for now, as changing
them is more involved (and they're linked to the allowTipSHA1InWant
config, which as a v0-only thing actually is always sha1).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>