Fix duplication of items in feeds.

Fix duplication of items in feeds.

When we request any part of the contents of a feed from the server, it
provides us with followup links we can use to extend the same feed
further into the past or future. But if we're extending it further
into the past than we had, we shouldn't save the future link we get
back, because it's not as good as the future link we already had. And
vice versa.

The effect of this bug was that after extending a feed into the past,
the next extension into the future would start from the past
fragment's future extension and re-retrieve all the items we already
had. And we wouldn't notice, and would store a second copy of all
their ids in our list.