builtin/config: move display options into local variables

The display options are tracked via a set of global variables. Move
them into a self-contained structure so that we can easily parse all
relevant options and hand them over to the various functions that
require them.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t