X wcfs/xbtree: Fix update not to add duplicate extra point if rebuild  - called by Update - already added it

It was failing e.g. as

=== RUN   TestΔBTail/rebuild/T4/T1,3-T/T-T-T-T/B0:b-B1:c,2:j-T-B4:d/B3:h→T/T2,3/T-T-T/B1:d-B2:c-B3:i/_T{3};R/_→T2/B1:g-B2:c,3:i
    δbtail_test.go:917: after Update(@at1→@at2): vδT:
        have:
        	@at1: map[0:{b ø} 1:{c d} 2:{j c} 3:{h i} 4:{d ø}]
        	@at2: map[1:{d g}]
        vδb/root:
        	@at1
        	@at2
        	@at2	<-- HERE