avoid storing same filename repeatedly in versioned import from S3

avoid storing same filename repeatedly in versioned import from S3

Logically, this should make it need a lot less memory when files have
been changed many times. In my tests, it didn't seem to change memory
use at all. Unsure why, it is working. It's possible the Response is not
getting garbage collected due to pinning. But as far as I can see, all
parts of it that are retained get copied in a way that won't keep the
whole thing pinned in memory.