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>

config.h

@@ -116,12 +116,14 @@ struct key_value_info {
 	int linenr;
 	enum config_origin_type origin_type;
 	enum config_scope scope;
+	const char *path;
 };
 #define KVI_INIT { \
 	.filename = NULL, \
 	.linenr = -1, \
 	.origin_type = CONFIG_ORIGIN_UNKNOWN, \
 	.scope = CONFIG_SCOPE_UNKNOWN, \
+	.path = NULL, \
 }
 
 /* Captures additional information that a config callback can use. */