simplestore: back up index when adding a revision This ensures that transaction rollback reverts the index to its previous state. This fixed a few test failures due to `hg verify` complaining about a reference to an undefined changeset revision. Differential Revision: https://phab.mercurial-scm.org/D3104

tests: disable shallow narrow tests with simple store I think these are failing because of issues with flags processing in the simple store. Let's revisit this later. Differential Revision: https://phab.mercurial-scm.org/D3103