See changes in δbtail.go for overview.

* t2+RebuildJob: (52 commits)
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  X Track should be nop if keycov/path is already in krebuildJobs
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  X xbtree/blib: RangedMap, RangedSet += IntersectsRange, Intersection
  X xbtree: tests: Also verify state of ΔTtail.ktrackNew
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IntersectsRange is needed for one assert in concurrenct ΔBtail rebuild.

Triggered one bug in ΔFtail.SliceByFileRev which was not checking
file's root to be VDEL.