mktag tests: don't needlessly use a subshell

The use of a subshell dates back to e9b20943b7 (t/t3800: do not use a
temporary file to hold expected result., 2008-01-04). It's not needed
anymore, if it ever was.

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

t/t3800-mktag.sh

@@ -14,7 +14,7 @@ test_description='git mktag: tag object verify test'
 check_verify_failure () {
 	expect="$2"
 	test_expect_success "$1" '
-		( test_must_fail git mktag <tag.sig 2>message ) &&
+		test_must_fail git mktag <tag.sig 2>message &&
 		grep "$expect" message
 	'
 }