config: pass source to config_parser_event_fn_t

config: pass source to config_parser_event_fn_t

..so that the callback can use a "struct config_source" parameter
instead of "config_reader.source". "struct config_source" is internal to
config.c, so we are adding a pointer to a struct defined in config.c
into a public function signature defined in config.h, but this is okay
because this function has only ever been (and probably ever will be)
used internally by config.c.

As a result, the_reader isn't used anywhere, so "struct config_reader"
is obsolete (it was only intended to be used with the_reader). Remove
them.

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