t1400 (update-ref): use test_must_fail

As t/README explains:

	When a gitcommand dies due to a segfault, test_must_fail
	diagnoses it as an error; "! git <command>" treats it as
	just another expected failure, which would let such a bug
	go unnoticed.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1400-update-ref.sh

@@ -52,9 +52,8 @@ rm -f .git/$m
 
 test_expect_success \
 	"fail to create $n" \
-	"touch .git/$n_dir
-	 git update-ref $n $A >out 2>err"'
-	 test $? != 0'
+	"touch .git/$n_dir &&
+	 test_must_fail git update-ref $n $A >out 2>err"
 rm -f .git/$n_dir out err
 
 test_expect_success \