dirstate: make sure the dirstate is loaded before the changelog (issue6303) Before this change, it was possible for the changelog to be loaded before the dirstate. If a transaction happens betwen the changelog and dirstate reading, the dirstate can up end poitning toward a revision not existing in the (olded) changelog. This lead to a warning. With this revision, we preload the dirstate parent before reading the changelog. This has a negligible performance impact on performance for all case we are tracking. Differential Revision: https://phab.mercurial-scm.org/D8528