Provide git_config with a callback-data parameter

git_config() only had a function parameter, but no callback data
parameter.  This assumes that all callback functions only modify
global variables.

With this patch, every callback gets a void * parameter, and it is hoped
that this will help the libification effort.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-file.c

@@ -31,7 +31,7 @@ int main(int argc, char **argv)
 		die("Not a valid object name %s", argv[1]);
 
 	setup_git_directory();
-	git_config(git_default_config);
+	git_config(git_default_config, NULL);
 
 	puts(create_temp_file(sha1));
 	return 0;