Crash on symlinks involving Unicode characters

This is split and simplified from bug #319313.

If there is a SVN repository with symlinks involving Unicode characters, it crashes bzr-svn.

Reproduction:

svnadmin create repos
svn co file://`pwd`/repos wc
cd wc
touch 𠀀
ln -s 𠀀 a
touch b
ln -s b 𠀁
svn add *
svn commit -m "Added files and links"
bzr up
bzr: ERROR: exceptions.UnicodeEncodeError: 'ascii' codec can't encode character u'\U00020001' in position 47: ordinal not in range(128)

Traceback (most recent call last):
  File "/home/wjlanda/lib/python/bzrlib/commands.py", line 893, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/home/wjlanda/lib/python/bzrlib/commands.py", line 839, in run_bzr
    ret = run(*run_argv)
  File "/home/wjlanda/lib/python/bzrlib/commands.py", line 539, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/wjlanda/lib/python/bzrlib/builtins.py", line 1189, in run
    tree = WorkingTree.open_containing(dir)[0]
  File "/home/wjlanda/lib/python/bzrlib/workingtree.py", line 333, in open_containing
    return control.open_workingtree(), relpath
  File "/home/wjlanda/.bazaar/plugins/svn/workingtree.py", line 830, in open_workingtree
    return SvnWorkingTree(self, self.local_path, self.open_branch())
  File "/home/wjlanda/.bazaar/plugins/svn/workingtree.py", line 114, in __init__
    self._update_base_revnum(max_rev)
  File "/home/wjlanda/.bazaar/plugins/svn/workingtree.py", line 551, in _update_base_revnum
    self.read_working_inventory()
  File "/home/wjlanda/.bazaar/plugins/svn/workingtree.py", line 422, in read_working_inventory
    add_dir_to_inv(u"", rootwc, None)
  File "/home/wjlanda/.bazaar/plugins/svn/workingtree.py", line 416, in add_dir_to_inv
    add_file_to_inv(subrelpath, subid, subrevid, id)
  File "/home/wjlanda/.bazaar/plugins/svn/workingtree.py", line 307, in add_file_to_inv
    file.symlink_target = os.readlink(self.abspath(relpath))
UnicodeEncodeError: 'ascii' codec can't encode character u'\U00020001' in position 47: ordinal not in range(128)

bzr 1.12dev on python 2.5.2 (linux2)
arguments: ['/home/wjlanda/bin/bzr', 'up']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_US.UTF-8'
plugins:
  bisect /home/wjlanda/.bazaar/plugins/bisect [1.1.0.pre.0]
  bzrtools /usr/lib/python2.5/site-packages/bzrlib/plugins/bzrtools [1.11]
  cvsps_import /usr/lib/python2.5/site-packages/bzrlib/plugins/cvsps_import [unknown]
  gtk /home/wjlanda/.bazaar/plugins/gtk [0.96.0.dev.1]
  launchpad /home/wjlanda/lib/python/bzrlib/plugins/launchpad [unknown]
  loom /usr/lib/python2.5/site-packages/bzrlib/plugins/loom [1.4dev]
  netrc_credential_store /home/wjlanda/lib/python/bzrlib/plugins/netrc_credential_store [unknown]
  qbzr /home/wjlanda/.bazaar/plugins/qbzr [0.9.7dev]
  rebase /usr/lib/python2.5/site-packages/bzrlib/plugins/rebase [0.4.2]
  search /usr/lib/python2.5/site-packages/bzrlib/plugins/search [1.7dev]
  stats /usr/lib/python2.5/site-packages/bzrlib/plugins/stats [unknown]
  svn /home/wjlanda/.bazaar/plugins/svn [0.5rc1]
  upload /usr/lib/python2.5/site-packages/bzrlib/plugins/upload [0.1]
*** Bazaar has encountered an internal error.
    Please report a bug at https://bugs.launchpad.net/bzr/+filebug
    including this traceback, and a description of what you
    were doing when the error occurred.