config: pass ctx with config files

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>