Fix corner case with recent narrowing optimization.
Fix corner case with recent narrowing optimization.

For a commit that was just merged I had the "back-out" case
at the wrong nesting level.  It was a pretty obscure failure
scenario that never came up in practice, but basically if you
were starting at a message that was not in your narrow, but
we did have some messages in your narrow, we would try to
go near the old message instead of talking to the server to
find the next unread message in that narrow.

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