update: job ID must be positive

The documentation for directory_update_init() was incorrect: a job ID
must be positive, not non-negative.  If the update queue is full and
no job was created, it makes more sense to return 0 instead of -1,
because it is more consistent with the return value of isUpdatingDB().

src/update.c

@@ -436,9 +436,9 @@ int directory_update_init(char *path)
 		int next_task_id;
 
 		if (!path)
-			return -1;
+			return 0;
 		if (update_paths_nr == ARRAY_SIZE(update_paths))
-			return -1;
+			return 0;
 		assert(update_paths_nr < ARRAY_SIZE(update_paths));
 		update_paths[update_paths_nr++] = path;
 		next_task_id = update_task_id + update_paths_nr;