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/credential.c

@@ -6,7 +6,10 @@
 static const char usage_msg[] =
 	"git credential (fill|approve|reject)";
 
-int cmd_credential(int argc, const char **argv, const char *prefix UNUSED)
+int cmd_credential(int argc,
+		   const char **argv,
+		   const char *prefix UNUSED,
+		   struct repository *repo UNUSED)
 {
 	const char *op;
 	struct credential c = CREDENTIAL_INIT;