deltas: skip if projected compressed size does not match text size constraint If we have a delta, we check constraints against a lower bound estimate of the resulting compressed delta. We then checks this projected size against the ½ⁿ size constraints. This allows to exclude potential base candidates before doing any expensive computation. This only apply to the intermediate-snapshot case since this constraint only apply to them. For some pathological cases of a private repository this step provide a further performance boost (timing from `hg perfrevlogwrite`): before: 3.145906 seconds after: 3.010646 seconds

deltas: accept and skip None return for delta info They are some extra computation that will shortcut the delta compression if the delta seems hopeless, returning None.