config: use an enum for type

config: use an enum for type

The --type=<X> option for 'git config' has previously been defined using
macros, but using a typed enum is better for tracking the possible
values.

Move the definition up to make sure it is defined before a macro uses
some of its terms.

Update the initializer for config_display_options to explicitly set
'type' to TYPE_NONE even though this is implied by a zero value.

This assists in knowing that the switch statement added in the previous
change has a complete set of cases for a properly-valued enum.

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