shallow: add repository argument to register_shallow

Add a repository argument to allow callers of register_shallow
to be more specific about which repository to handle. This is a small
mechanical change; it doesn't change the implementation to handle
repositories other than the_repository yet.

As with the previous commits, use a macro to catch callers passing a
repository other than the_repository at compile time.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -2875,7 +2875,7 @@ static void get_object_list(int ac, const char **av)
 				struct object_id oid;
 				if (get_oid_hex(line + 10, &oid))
 					die("not an SHA-1 '%s'", line + 10);
-				register_shallow(&oid);
+				register_shallow(the_repository, &oid);
 				use_bitmap_index = 0;
 				continue;
 			}