autogen.sh: don't print old git-config values

Old values are of no interest to the user, so let's reduce the spam
a bit by hiding those.

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Chad Versace <chadversary@chromium.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>