t3903: fix misquoted rev-parse invocation

!"git ..." hopefully always succeeds because "git ..." is not the name
of any executable.  However, that's not what was intended.  Unquote
it, and while we're at it, also replace ! with test_must_fail since it
is a call to git.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3903-stash.sh

@@ -461,7 +461,7 @@ test_expect_success 'ref with non-existant reflog' '
 	echo bar6 > file2 &&
 	git add file2 &&
 	git stash &&
-	! "git rev-parse --quiet --verify does-not-exist" &&
+	test_must_fail git rev-parse --quiet --verify does-not-exist &&
 	test_must_fail git stash drop does-not-exist &&
 	test_must_fail git stash drop does-not-exist@{0} &&
 	test_must_fail git stash pop does-not-exist &&