t6120 (describe): check --long properly

Existing test checked --long only for exactly tagged commit.  We should
make sure it works sensibly for commits that are not tagged.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6120-describe.sh

@@ -98,6 +98,7 @@ check_describe A-* --tags HEAD^^2
 check_describe B --tags HEAD^^2^
 
 check_describe B-0-* --long HEAD^^2^
+check_describe A-3-* --long HEAD^^2
 
 test_expect_success 'rename tag A to Q locally' '
 	mv .git/refs/tags/A .git/refs/tags/Q