t0612: mark as leak-free

t0612: mark as leak-free

A quick test tells us that t0612 does not trigger any leak:

    $ make SANITIZE=leak test GIT_TEST_PASSING_SANITIZE_LEAK=check GIT_TEST_SANITIZE_LEAK_LOG=true GIT_TEST_OPTS=-i T=t0612-reftable-jgit-compatibility.sh
    [...]
    *** t0612-reftable-jgit-compatibility.sh ***
    in GIT_TEST_PASSING_SANITIZE_LEAK=check mode, setting --invert-exit-code for TEST_PASSES_SANITIZE_LEAK != true
    ok 1 - CGit repository can be read by JGit
    ok 2 - JGit repository can be read by CGit
    ok 3 - mixed writes from JGit and CGit
    ok 4 - JGit can read multi-level index
    # passed all 4 test(s)
    1..4
    # faking up non-zero exit with --invert-exit-code
    make[2]: *** [Makefile:75: t0612-reftable-jgit-compatibility.sh] Error 1

Let's mark it as leak-free to silence the machinery activated by
`GIT_TEST_PASSING_SANITIZE_LEAK=check`.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>