object-name tests: add tests for ambiguous object blind spots

object-name tests: add tests for ambiguous object blind spots

Extend the tests for ambiguous objects to check how we handle objects
where we return OBJ_BAD when trying to parse them. As noted in [1] we
have a blindspot when it comes to this behavior.

Since we need to add new test data here let's extend these tests to be
tested under SHA-256, in d7a2fc82491 (t1512: skip test if not using
SHA-1, 2018-05-13) all of the existing tests were skipped, as they
rely on specific SHA-1 object IDs.

For these tests it only matters that the first 4 characters of the OID
prefix are the same for both SHA-1 and SHA-256. This uses strings that
I mined, and have the same prefix when hashed with both.

We "test_cmp" the full output to guard against any future regressions,
and because a subsequent commit will tweak it. Showing a diff of how
the output changes is helpful to explain those subsequent commits.

The "sed" invocation in test_cmp_failed_rev_parse() doesn't need a
"/g" because under both SHA-1 and SHA-256 we'll wildcard match any
trailing part of the OID after our known starting prefix. We'd like to
convert all of that to just "..." for the "test_cmp" which follows.

1. https://lore.kernel.org/git/YZwbphPpfGk78w2f@coredump.intra.peff.net/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>