builtin: add a repository parameter for builtin functions

In order to reduce the usage of the global the_repository, add a
parameter to builtin functions that will get passed a repository
variable.

This commit uses UNUSED on most of the builtin functions, as subsequent
commits will modify the actual builtins to pass the repository parameter
down.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout--worker.c

@@ -113,7 +113,10 @@ static const char * const checkout_worker_usage[] = {
 	NULL
 };
 
-int cmd_checkout__worker(int argc, const char **argv, const char *prefix)
+int cmd_checkout__worker(int argc,
+			 const char **argv,
+			 const char *prefix,
+			 struct repository *repo UNUSED)
 {
 	struct checkout state = CHECKOUT_INIT;
 	struct option checkout_worker_options[] = {