Implement 'svnmerge hidden'.
'svnmerge hidden' will list all the revisions marked as blocked, against
the default or a given head (given by the -S/--head options).

* contrib/client-side/svnmerge.py
  (action_hidden): New function to handle the 'svnmerge hidden' command.
  (command_table): Added new Option for handling the 'hidden' subcommand.