upgrade: split some logic from UpgradeOperation The logic for automatic-upgrade and the upgrade-repo should be able to use the same code. However that code often need an UpgradeOperation object to function. So we start spliting the Operation into a minimal component that we will be able to reuse outside of the "classic" upgrade path. We will put the base-class to use in the next changeset. Differential Revision: https://phab.mercurial-scm.org/D12612