t6700: mark test as leak-free

This test has never leaked since it was added. Let's annotate it to make
sure it stays that way (and to reduce noise when looking for other
leak-free scripts after we fix some leaks).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6700-tree-depth.sh

@@ -1,6 +1,8 @@
 #!/bin/sh
 
 test_description='handling of deep trees in various commands'
+
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 # We'll test against two depths here: a small one that will let us check the