t6200: use test_config/test_unconfig

The tests were already well protected from previous ones by running
"git config --unset" on variables early they do not want to see, but
it is easier to make sure they start from a clean state by using
more modern test_config/test_unconfig helper functions.

It turns out that the last test depended on the merge.summary
configuration previous one leaves behind.  Set it explicitly in it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
t6200: test message for merging of an annotated tag

Signed-off-by: Junio C Hamano <gitster@pobox.com>