Use ΔValue.Old for closest to tail entry instead.

=== RUN   TestΔFtail
2021/07/09 15:55:51 zodb: FIXME: open /tmp/TestΔFtail2848740640/001/1.fs: raw cache is not ready for invalidations -> NoCache forced
    δftail_test.go:176: # → @at1 (03e14ec7dc3deb22)  δT1:a,2:b,3:ø  δD{a}       ; t1:a,2:b
    δftail_test.go:176: # → @at2 (03e14ec7dc3f06ee)  δTø  δD{c} ; t1:a,2:b
    δftail_test.go:176: # → @at3 (03e14ec7dc3fcdbb)  δT2:c  δD{a b}     ; t1:a,2:c
    δftail_test.go:176: # → @at4 (03e14ec7dc40c888)  δT5:i  δD{}        ; t1:a,2:c,5:i
    δftail_test.go:176: # → @at5 (03e14ec7dc41beee)  δT5:e  δD{}        ; t1:a,2:c,5:e
    δftail_test.go:176: # → @at6 (03e14ec7dc429d33)  δTø  δD{i} ; t1:a,2:c,5:e
    δftail_test.go:194: wrong δF.ByFile:
        have keys: {000000000000000e}
        want keys: {}
    δftail_test.go:176: # → @at7 (03e14ec7dc4377dd)  δT1:a,6:i,7:d,8:e  δD{a c e f g h i j}     ; t1:a,2:c,5:e,6:i,7:d,8:e
--- FAIL: TestΔFtail (0.71s)