mercurial/transaction.py

--- a/mercurial/transaction.py	Thu Dec 18 12:07:03 2014 -0600
+++ b/mercurial/transaction.py	Mon Dec 15 13:27:46 2014 -0800
@@ -200,8 +200,8 @@
             return
         dirname, filename = os.path.split(file)
         backupfilename = "%s.backup.%s" % (self.journal, filename)
-        backupfile = os.path.join(dirname, backupfilename)
         vfs = self._vfsmap[location]
+        backupfile = vfs.reljoin(dirname, backupfilename)
         if vfs.exists(file):
             filepath = vfs.join(file)
             backuppath = vfs.join(backupfile)