index-pack tests: don't leave test repo dirty at end

Change a test added in 51054177b3 ("index-pack: detect local
corruption in collision check", 2017-04-01) so that the repository
isn't left dirty at the end.

Due to the caveats explained in 720dae5a19 ("config doc: elaborate on
fetch.fsckObjects security", 2018-07-27) even a "fetch" that fails
will write to the local object store, so let's copy the bit-error test
directory before running this test.

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

t/t1060-object-corruption.sh

@@ -117,8 +117,10 @@ test_expect_failure 'clone --local detects misnamed objects' '
 '
 
 test_expect_success 'fetch into corrupted repo with index-pack' '
+	cp -R bit-error bit-error-cp &&
+	test_when_finished "rm -rf bit-error-cp" &&
 	(
-		cd bit-error &&
+		cd bit-error-cp &&
 		test_must_fail git -c transfer.unpackLimit=1 \
 			fetch ../no-bit-error 2>stderr &&
 		test_i18ngrep ! -i collision stderr