config.c: remove current_config_kvi

config.c: remove current_config_kvi

Add ".config_kvi" to "struct config_reader" and replace
"current_config_kvi" with "the_reader.config_kvi", plumbing "struct
config_reader" where necesssary.

Also, introduce a setter function for ".config_kvi", which allows us to
enforce the contraint that only one of ".source" and ".config_kvi" can
be set at a time (as documented in the comments). Because of this
constraint, we know that "populate_remote_urls()" was never touching
"current_config_kvi" when iterating through config files, so it doesn't
need to store and restore that value.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>