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/diff.c

@@ -393,7 +393,10 @@ static void symdiff_release(struct symdiff *sdiff)
 	bitmap_free(sdiff->skip);
 }
 
-int cmd_diff(int argc, const char **argv, const char *prefix)
+int cmd_diff(int argc,
+	     const char **argv,
+	     const char *prefix,
+	     struct repository *repo UNUSED)
 {
 	int i;
 	struct rev_info rev;