repo-settings: consolidate some config settings

repo-settings: consolidate some config settings

There are a few important config settings that are not loaded
during git_default_config. These are instead loaded on-demand.

Centralize these config options to a single scan, and store
all of the values in a repo_settings struct. The values for
each setting are initialized as negative to indicate "unset".

This centralization will be particularly important in a later
change to introduce "meta" config settings that change the
defaults for these config settings.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>