fsck tests: refactor one test to use a sub-repo

fsck tests: refactor one test to use a sub-repo

Refactor one of the fsck tests to use a throwaway repository. It's a
pervasive pattern in t1450-fsck.sh to spend a lot of effort on the
teardown of a tests so we're not leaving corrupt content for the next
test.

We can instead use the pattern of creating a named sub-repository,
then we don't have to worry about cleaning up after ourselves, nobody
will care what state the broken "hash-mismatch" repository is after
this test runs.

See [1] for related discussion on various "modern" test patterns that
can be used to avoid verbosity and increase reliability.

1. https://lore.kernel.org/git/87y27veeyj.fsf@evledraar.gmail.com/

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