config: add kvi.path, use it to evaluate includes

config: add kvi.path, use it to evaluate includes

Include directives are evaluated using the path of the config file. To
reduce the dependence on "config_reader.source", add a new
"key_value_info.path" member and use that instead of
"config_source.path". This allows us to remove a "struct config_reader
*" field from "struct config_include_data", which will subsequently
allow us to remove "struct config_reader" entirely.

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