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.

snapshot: refine candidate snapshot base upward Once we found a suitable snapshot base it is useful to check if it has a "children" snapshot that would provide a better diff. This is useful when base not directly related to stored revision are picked. In those case, we "jumped" to this new chain at an arbitrary point, checking if a higher point is more appropriate will help to provide better results and increase snapshot reuse.

snapshot: try to refine new snapshot base down the chain There are cases where doing a diff against a snapshot's parent will be shorter than against the snapshot itself. Reusing snapshot not directly related to the revision we are trying to store increase this odd. So once we found a possible candidate, we check the snapshots lower in the chain. This will involve extra processing, but this extra processing will only happen when we are doing building a snapshot, a rare situation.