t1450: use "mv -f" within loose object directory

The loose objects are created with mode 0444. That doesn't prevent them being overwritten by rename(), but some versions of "mv" will be extra careful and prompt the user, even without "-i". Reportedly macOS does this, at least in the Travis builds. The prompt reads from /dev/null, defaulting to "no", and the object isn't moved. Then to make matters even more interesting, it still returns "0" and the rest of the test proceeds, but with a broken setup. We can work around it by using "mv -f" to override the prompt. This should work as it's already used in t5504 for the same purpose. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>