We need to test this because the following case happens often in
practice: the tree did not exist when ΔBtail was started, later the tree
is created and ΔBtail is recomputed. Since we are testing tree transitions
that involves delete, test it uniformly for all cases.

Currently Update test, if Fatal("TODO") is removed fails with:

    --- FAIL: TestΔBTail/Update/T2,5/B1:a-B2:b,4:d-B8:h→ø (0.00s)
panic: ΔBtail update 03e036b7f59f86cc -> 03e036b7f5a28899: treediff 03e036b7f59f86cc..03e036b7f5a28899 000000000000000c: diffT 03e036b7f59f86cc:000000000000000c 03e036b7f5a28899:000000000000000c: BTrees.LOBTree.LOBTree(000000000000000c): activate: file:///tmp/δBTail851157667/1.fs: load 03e036b7f5a28899:000000000000000c: 000000000000000c: object was deleted @03e036b7f5a28899 [recovered]