config: remove unneeded struct field

As well as receiving the config key and value, config callbacks
also receive a "struct key_value_info" containing information about
the source of the key-value pair. Accessing the "path" field of
this struct from a callback passed to repo_config() results in a
use-after-free. This happens because repo_config() first populates a
configset by calling config_with_options() and then iterates over the
configset with the callback passed by the caller. When the configset
is constructed it takes a shallow copy of the "struct key_value_info"
for each config setting. This leads to the use-after-free as the
"path" member is freed before config_with_options() returns.

We could fix this by interning the "path" field as we do
for the "filename" field but the "path" field is not actually
needed. It is populated with a copy of the "path" field from "struct
config_source". That field was added in d14d42440d (config: disallow
relative include paths from blobs, 2014-02-19) to distinguish between
relative include directives in files and those in blobs. However,
since 1b8132d99d (i18n: config: unfold error messages marked for
translation, 2016-07-28) we can differentiate these by looking at the
"origin_type" field in "struct key_value_info". So let's remove the
"path" members from "struct config_source" and "struct key_value_info"
and instead use a combination of the "filename" and "origin_type"
fields to determine the absolute path of relative includes.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.h

@@ -122,14 +122,12 @@ 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. */