config.c: pass ctx in configsets

config.c: pass ctx in configsets

Pass config_context to config callbacks in configset_iter(), trivially
setting the .kvi member to the cached key_value_info. Then, in config
callbacks that are only used with configsets, use the .kvi member to
replace calls to current_config_*(), and delete current_config_line()
because it has no remaining callers.

This leaves builtin/config.c and config.c as the only remaining users of
current_config_*().

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