t3200: unset core.logallrefupdates when testing reflog creation

This test checks that the "-l" option creates a reflog. But in fact we'd create one even without it, since the default in a non-bare repository is to do so. Let's unset the config so we can be sure our "-l" option is kicking in. Note that we can't do this with test_config, since that would leave the variable unset after our test finishes, confusing downstream tests (the helper is not not smart enough to restore the previous value, and just always runs test_unconfig). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>