fix crash in mantash listing at login level

[trent.mick@manta /trent.mick]$ ls
mantash: ERROR: list index out of range (/Users/trentm/joy/python-manta/bin/mantash:594 in _visited_known_mpath)

Traceback (most recent call last):
  File "/Users/trentm/joy/python-manta/bin/mantash", line 1725, in <module>
    retval = main(sys.argv)
  File "/Users/trentm/joy/python-manta/bin/mantash", line 1719, in main
    return shell.main(argv, loop=cmdln.LOOP_IF_EMPTY)
  File "/Users/trentm/joy/python-manta/manta/cmdln.py", line 268, in main
    return self.cmdloop()
  File "/Users/trentm/joy/python-manta/manta/cmdln.py", line 358, in cmdloop
    retval = self.onecmd(argv)
  File "/Users/trentm/joy/python-manta/manta/cmdln.py", line 422, in onecmd
    return self._dispatch_cmd(handler, argv)
  File "/Users/trentm/joy/python-manta/manta/cmdln.py", line 1125, in _dispatch_cmd
    return handler(argv[0], opts, *args)
  File "/Users/trentm/joy/python-manta/bin/mantash", line 512, in do_ls
    to_print = self._ls_path(path, opts.dir)
  File "/Users/trentm/joy/python-manta/bin/mantash", line 480, in _ls_path
    self._visited_known_mpath(abspath)
  File "/Users/trentm/joy/python-manta/bin/mantash", line 594, in _visited_known_mpath
    area = mpath.split('/')[2]
IndexError: list index out of range