t5520: use test_config to set/unset git config variables (leftover bits)

Configuration from test_config does not last beyond the end of the
current test assertion, making each test easier to think about in
isolation.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>