Since we are opening each directory for iteration, we know that we don't need to reset the cursor's directory before iterating. Using `iterateAssumeFirstIteration` skips the cursor resetting which eliminates an `lseek` syscall for every directory opened on non-Windows platforms.

This doesn't seem to actually matter much for performance (1.01 ± 0.02 times faster when walking /home/ on my system) but avoiding unnecessary syscalls is always nice anyway.