[BUG] stgit branch renaming into new dir crashes

When trying to rename a branch to a name including a slash, there is no explicit creation of leading dirs, and stgit crashes:

$ stg branch -r multitag dev/multitag
Traceback (most recent call last):
  File "/usr/bin/stg", line 43, in ?
    main()
  File "/usr/lib/python2.3/site-packages/stgit/main.py", line 187, in main
    command.func(parser, options, args)
  File "/usr/lib/python2.3/site-packages/stgit/commands/branch.py", line 214, in func
    stack.Series(args[0]).rename(args[1])
  File "/usr/lib/python2.3/site-packages/stgit/stack.py", line 497, in rename
    os.rename(self.__series_dir, to_stack.__series_dir)
OSError: [Errno 2] No such file or directory