config: pass ctx with config files

Pass config_context to config_callbacks when parsing config files. To
provide the .kvi member, refactor out the configset logic that caches
"struct config_source" and "enum config_scope" as a "struct
key_value_info". Make the "enum config_scope" available to the config
file machinery by plumbing an additional arg through
git_config_from_file_with_options().

We do not exercise ctx yet because the remaining current_config_*()
callers may be used with config_with_options(), which may read config
from parameters, but parameters don't pass ctx yet.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/fsck.c b/fsck.c
index 55b6a69..f92c216 100644
--- a/fsck.c
+++ b/fsck.c
@@ -1238,7 +1238,8 @@ static int fsck_blob(const struct object_id *oid, const char *buf,
 		data.ret = 0;
 		config_opts.error_action = CONFIG_ERROR_SILENT;
 		if (git_config_from_mem(fsck_gitmodules_fn, CONFIG_ORIGIN_BLOB,
-					".gitmodules", buf, size, &data, &config_opts))
+					".gitmodules", buf, size, &data,
+					CONFIG_SCOPE_UNKNOWN, &config_opts))
 			data.ret |= report(options, oid, OBJ_BLOB,
 					FSCK_MSG_GITMODULES_PARSE,
 					"could not parse gitmodules blob");