bzr mv doesn't gracefully handle symlinks to directories in paths

bzr mv attempts to find the children of a symlink when decoding a source path. This is probably a more general bug.

It seems as though we should
1. Give a better error (with traceback) when path2id attempts to find the children of a symlink
2. Determine the real path of source and target in 'bzr mv' before attempting to do path2id.

~$ bzr init foo
abentley@troll:~$ cd foo
abentley@troll:~/foo$ mkdir bar
abentley@troll:~/foo$ ln -s bar baz
abentley@troll:~/foo$ touch baz/qux
abentley@troll:~/foo$ bzr add
added bar
added baz
added bar/qux
abentley@troll:~/foo$ bzr mv baz/qux ./qux
bzr: ERROR: exceptions.AttributeError: children

Traceback (most recent call last):
  File "/home/abentley/bzr/dev/bzrlib/commands.py", line 623, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/home/abentley/bzr/dev/bzrlib/commands.py", line 585, in run_bzr
    ret = run(*run_argv)
  File "/home/abentley/bzr/dev/bzrlib/commands.py", line 291, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/abentley/bzr/dev/bzrlib/builtins.py", line 451, in run
    tree.rename_one(rel_names[0], rel_names[1])
  File "/home/abentley/bzr/dev/bzrlib/mutabletree.py", line 34, in tree_write_locked
    return unbound(self, *args, **kwargs)
  File "/home/abentley/bzr/dev/bzrlib/workingtree.py", line 1075, in rename_one
    file_id = inv.path2id(from_rel)
  File "/home/abentley/bzr/dev/bzrlib/inventory.py", line 1207, in path2id
    cie = parent.children[f]
AttributeError: children

bzr 0.12.0dev0 on python 2.4.1.final.0 (linux2)
arguments: ['/home/abentley/bin/bzr', 'mv', 'baz/qux', './qux']

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