t1400: test one mistake at a time

This case wants to test passing a bad refname to the "update" command.
But it also passes too few arguments to "update", which muddles the
situation: which error should be diagnosed?  So split this test into
two:

* One that passes too few arguments to update

* One that passes all three arguments to "update", but with a bad
  refname.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1400-update-ref.sh

@@ -724,8 +724,14 @@ test_expect_success 'stdin -z fails update with no ref' '
 	grep "fatal: update line missing <ref>" err
 '
 
+test_expect_success 'stdin -z fails update with too few args' '
+	printf $F "update $a" "$m" >stdin &&
+	test_must_fail git update-ref -z --stdin <stdin 2>err &&
+	grep "fatal: update $a missing <oldvalue>" err
+'
+
 test_expect_success 'stdin -z fails update with bad ref name' '
-	printf $F "update ~a" "$m" >stdin &&
+	printf $F "update ~a" "$m" "" >stdin &&
 	test_must_fail git update-ref -z --stdin <stdin 2>err &&
 	grep "fatal: invalid ref format: ~a" err
 '