mktag tests: remove needless SHA-1 hardcoding

Change the tests amended in acb49d1cc8b (t3800: make hash-size independent, 2019-08-18) even more to make them independent of either SHA-1 or SHA-256. Some of these tests were failing for the wrong reasons. The first one being modified here would fail because the line starts with "xxxxxx" instead of "object", the rest of the line doesn't matter. Let's just put a valid hash on the rest of the line anyway to narrow the test down for just the s/object/xxxxxx/ case. The second one being modified here would fail under GIT_TEST_DEFAULT_HASH=sha256 because <some sha-1 length garbage> is an invalid SHA-256, but we should really be testing <some sha-256 length garbage> when under SHA-256. This doesn't really matter since we should be able to trust other parts of the code to validate things in the 0-9a-f range, but let's keep it for good measure. There's a later test which tests an invalid SHA which looks like a valid one, to stress the "We refuse to tag something we can't verify[...]" logic in mktag.c. But here we're testing for a SHA-length string which contains characters outside of the /[0-9a-f]/i set. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>