strip: explicitly compute the boundary of the backup bundle
We want to make change to the set of backed up revision in a future changeset,
we start with a change of the computation without any changes in the semantic
to clarify later changeset.
The could of costly assert are here to testify that the result is still
correct. They will be removed in the next changesets, but I wanted them in this
changeset to help in case someone bisect a regression to this changeset in the
future.
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