snapshot: also consider the snapshot chain of one unrelated revision To maximize the chance of good delta chain reuse, we inject an unrelated delta chain into our search. To do so, we search for the highest revision unrelated to the parents of the current revision and use its snapshot chain too. Adding this extra snapshot into the mix can have a performance impact. We'll deal with performance impact in a later series.

snapshot: extract parent chain computation The final step of this series is to include chain related to "prev" in the search. Before adding that code we do some simple code movement to clarify the next diff.