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/init-db.c

@@ -70,7 +70,10 @@ static const char *const init_db_usage[] = {
  * On the other hand, it might just make lookup slower and messier. You
  * be the judge.  The default case is to have one DB per managed directory.
  */
-int cmd_init_db(int argc, const char **argv, const char *prefix)
+int cmd_init_db(int argc,
+		const char **argv,
+		const char *prefix,
+		struct repository *repo UNUSED)
 {
 	const char *git_dir;
 	const char *real_git_dir = NULL;