tests: defang pager tests by explicitly disabling the log.mailmap warning

tests: defang pager tests by explicitly disabling the log.mailmap warning

In the previous patch, we added a deprecation warning for the current
log.mailmap setting. This warning only appears when git is attached to
a controlling terminal. Some tests however run under an emulated
terminal, so we need to disable the warning for those tests.

Thanks to Junio for suggesting that we do this in the setup function.

Signed-off-by: Ariadne Conill <ariadne@dereferenced.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>