I forgot to update go.sum entry there and it started to fail to build.
Thanks to Rafael for the bug report.

This will, hopefully, be used to fix rebuild.

* t2:
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  X treediff ret += δtkeycov
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Hopefully rebuild will use this to correctly recompute trackSet

As a consequence rework treediff to work on key ranges instead of
individial keys.

Adjacency coverage gets wider, but that is ok, because in practice if
keys are added to a BTree - that means that something is added to
ZBigFile, and it is very likely that just added data will be accessed
again. In other words it means that wcfs will very likely track those
data on nearby future read access, so we don't loose anything if we
start to track those keys right at the beginning.

* t2: (74 commits)
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  X kadj must be taken into account as kadj^δZ
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  ...