TieredMergePolicy's handling of the case that all segments are less than the floor segment size is fragile

In the case that the index size is less than the floor segment size, the allowed number of segments is always computed as 1. In practice, it doesn't keep merging indefinitely only because doFindMerges has some logic that skips merging if the number of candidates is less than maxMergeAtOnce. This looks a bit fragile to me.