config: change default of `pager.config` to "on"

config: change default of `pager.config` to "on"

This is similar to ff1e72483 (tag: change default of `pager.tag` to
"on", 2017-08-02) and is safe now that we do not consider `pager.config`
at all when we are not listing or getting configuration. This change
will help with listing large configurations, but will not hurt users of
`git config --edit` as it would have before the previous commit.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>