pack-objects tests: don't leave test .git corrupt at end

pack-objects tests: don't leave test .git corrupt at end

Change the pack-objects tests to not leave their .git directory
corrupt and the end.

In 2fca19fbb5 ("fix multiple issues with t5300", 2010-02-03) a comment
was added warning against adding any subsequent tests, but since
4614043c8f ("index-pack: use streaming interface for collision test on
large blobs", 2012-05-24) the comment has drifted away from the code,
mentioning two test, when we actually have three.

Instead of having this warning let's just create a new .git directory
specifically for these tests.

As an aside, it would be interesting to instrument the test suite to
run a "git fsck" at the very end (in "test_done"). That would have
errored before this change, and may find other issues #leftoverbits.

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