histedit: convert fold/roll actions into a class This converts the fold/roll actions into a histeditclass instance, as part of an ongoing effort to refactor histedit for maintainability and robustness. The tests changed for two reasons: 1) We get a new 'empty changeset' warning because we now warn more consistently between normal histedit and --continue about commits disappearing. 2) Previously we were not putting the histedit-source extra field on the temporary fold commit during normal runs, but we were on --continue runs. By unifying these code paths we now consistently put histedit-source on the temporary fold commit, which changes some of the hashes in the backup bundles.