tests: at-combinations: improve nonsense()

In some circumstances 'git log' might fail, but not because the @
parsing failed. For example: 'git rev-parse' might succeed and return a
bad object, and then 'git log' would fail.

The layer we want to test is revision parsing, so let's test that
directly.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1508-at-combinations.sh

@@ -18,7 +18,7 @@ check() {
 
 nonsense() {
 	test_expect_${2:-success} "$1 is nonsensical" "
-		test_must_fail git log -1 '$1'
+		test_must_fail git rev-parse --verify '$1'
 	"
 }