transaction: use the right location when cleaning up backup file (issue4479)

The location variable fetch from the loop and the one used to actually fetch it
mismatched. We fix the name to ensure file outside of store are cleaned up.

mercurial/transaction.py

@@ -397,7 +397,7 b' class transaction(object):'
             self.opener.unlink(self.journal)
         if self.opener.isfile(self._backupjournal):
             self.opener.unlink(self._backupjournal)
-            for _l, _f, b, c in self._backupentries:
+            for l, _f, b, c in self._backupentries:
                 if l not in self._vfsmap and c:
                     self.report("couldn't remote %s: unknown cache location"
                                 "%s\n" % (b, l))