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