tahoe backup should be able to backup symlinks

[francois]

Running tahoe backup on a directory containing a symbolic link currently doesn't work. It raises the following exception instead.

Traceback (most recent call last):
  File "/home/francois/dev/tahoe/support/bin/tahoe", line 8, in <module>
    load_entry_point('allmydata-tahoe==1.2.0-r3615', 'console_scripts', 'tahoe')()
  File "/home/francois/dev/tahoe/src/allmydata/scripts/runner.py", line 91, in run
    rc = runner(sys.argv[1:])
  File "/home/francois/dev/tahoe/src/allmydata/scripts/runner.py", line 78, in runner
    rc = cli.dispatch[command](so)
  File "/home/francois/dev/tahoe/src/allmydata/scripts/cli.py", line 359, in backup
    rc = tahoe_backup.backup(options)
  File "/home/francois/dev/tahoe/src/allmydata/scripts/tahoe_backup.py", line 353, in backup
    return bu.run()
  File "/home/francois/dev/tahoe/src/allmydata/scripts/tahoe_backup.py", line 198, in run
    new_backup_dircap = self.process(options.from_dir, latest_backup_dircap)
  File "/home/francois/dev/tahoe/src/allmydata/scripts/tahoe_backup.py", line 245, in process
    newchilddircap = self.process(childpath, oldchildcap)
  File "/home/francois/dev/tahoe/src/allmydata/scripts/tahoe_backup.py", line 245, in process
    newchilddircap = self.process(childpath, oldchildcap)
  File "/home/francois/dev/tahoe/src/allmydata/scripts/tahoe_backup.py", line 245, in process
    newchilddircap = self.process(childpath, oldchildcap)
  File "/home/francois/dev/tahoe/src/allmydata/scripts/tahoe_backup.py", line 251, in process
    raise RuntimeError("how do I back this up?" % childpath)
RuntimeError: how do I back this up?