transaction: really fix _addbackupentry key usage (issue4684)

The fix in fd905b2bea59 is actually wrong. We now use the filename to match what
'_addentry'. This whole untested code is quite suspicious. This seems to point
that no one is ever running 'tr.find' for a backup file.

mercurial/transaction.py

@@ -222,7 +222,7 class transaction(object):
     def _addbackupentry(self, entry):
         """register a new backup entry and write it to disk"""
         self._backupentries.append(entry)
-        self._backupmap[entry] = len(self._backupentries) - 1
+        self._backupmap[entry[1]] = len(self._backupentries) - 1
         self._backupsfile.write("%s\0%s\0%s\0%d\n" % entry)
         self._backupsfile.flush()