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