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/merge-base.c

@@ -143,7 +143,10 @@ static int handle_fork_point(int argc, const char **argv)
 	return 0;
 }
 
-int cmd_merge_base(int argc, const char **argv, const char *prefix)
+int cmd_merge_base(int argc,
+		   const char **argv,
+		   const char *prefix,
+		   struct repository *repo UNUSED)
 {
 	struct commit **rev;
 	int rev_nr = 0;