subrepo-git: use an official origvfs when appropriate

The origvfs has the auditor properly set and can move file without issue.

The current code is currently working without errors because rename are not
audited, yet.

mercurial/subrepo.py

@@ -1810,11 +1810,15 b' class gitsubrepo(abstractsubrepo):'
         if not opts.get(r'no_backup'):
             status = self.status(None)
             names = status.modified
+            origvfs = scmutil.getorigvfs(self.ui, self._subparent)
+            if origvfs is None:
+                origvfs = self.wvfs
             for name in names:
                 bakname = scmutil.origpath(self.ui, self._subparent, name)
                 self.ui.note(_('saving current version of %s as %s\n') %
                         (name, bakname))
-                self.wvfs.rename(name, bakname)
+                name = self.wvfs.join(name)
+                origvfs.rename(name, bakname)
 
         if not opts.get(r'dry_run'):
             self.get(substate, overwrite=True)