localrepo: use dirstate savebackup instead of handling dirstate file manually This is one step towards having dirstate manage its own storage. It will be useful for the implementation of sql dirstate [1]. This introduced a small test change: now we always write the dirstate before saving backup so in some cases where dirstate file didn't exist yet savebackup can create it. [1] https://www.mercurial-scm.org/wiki/SQLDirstatePlan