dirstate: update backup functions to take full backup filename Update the dirstate functions so that the caller supplies the full backup filename rather than just a prefix and suffix. The localrepo code was already hard-coding the fact that the backup name must be (exactly prefix + "dirstate" + suffix): it relied on this in _journalfiles() and undofiles(). Making the caller responsible for specifying the full backup name removes the need for the localrepo code to assume that dirstate._filename is always "dirstate". Differential Revision: https://phab.mercurial-scm.org/D68