Spurious fetch-older request when scrolling past a code block

[gnprice]

To reproduce:

1. Run the app in debug mode, and watch the debug console, where we print a log line for each HTTP request to the Zulip server.
2. Open the message list, in a narrow where there will be code blocks somewhere in the history, and plenty of older history before that.
3. Scroll until a code block comes into view. (Or more precisely: until a message containing a code block is nearly in view, so that its widget gets built.)
4. In the console, note the get-messages request for older messages.
5. Scroll the code block out of view (by enough so its widget gets removed), then back (nearly) into view, several times. Note the get-messages request each time the code block's message comes back (nearly) into view.

The expected behavior is that we should see such a request only if we've gotten within a few screenfuls of the top of the history we have. Step 4 could therefore happen by coincidence, but Step 5 should definitely not happen, because after the first such request we have plenty of previous history.

I discovered this while working on #80; it's a small oversight in how we're using a bit of the Flutter API. I'll send a fix.