t3200: unset core.logallrefupdates when testing reflog creation

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>