config.c: remove current_parsing_scope

config.c: remove current_parsing_scope

Add ".parsing_scope" to "struct config_reader" and replace
"current_parsing_scope" with "the_reader.parsing_scope. Adjust the
comment slightly to make it clearer that the scope applies to the config
source (not the current value), and should only be set when parsing a
config source.

As such, ".parsing_scope" (only set when parsing config sources) and
".config_kvi" (only set when iterating a config set) should not be
set together, so enforce this with a setter function.

Unlike previous commits, "populate_remote_urls()" still needs to store
and restore the 'scope' value because it could have touched
"current_parsing_scope" ("config_with_options()" can set the scope).

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