delta-find: introduce a _DeltaSearch object
That object represent the search of a good delta for one revision. It will
replace the interleaved generator currently in use. It will make the logic more
explicit and easier to split into different subclass for the algorithm variant.
We will move content gradually before doing deeper rework.
For now, we only move the `_candidategroups` function here. More will follow in
the same series.
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