t7060: actually test "git diff-index --cached -M"

A test was designed for "git diff-index --cached -M" but the command is
run without the "-M" option (which makes the test essentially identical
to its preceding counterpart).

Signed-off-by: Matthieu Prat <matthieuprat@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7060-wtstatus.sh

@@ -103,7 +103,7 @@ test_expect_success 'git diff-index --cached -M shows 2 added + 1 unmerged' '
 	A	THREE
 	A	TWO
 	EOF
-	git diff-index --cached --name-status HEAD >actual &&
+	git diff-index --cached -M --name-status HEAD >actual &&
 	test_cmp expected actual
 '