I was experiencing exceptions when using rbt diff --revision-range:
Traceback (most recent call last):

  File "/Users/gps/src/firefox/obj-firefox.noindex/_virtualenv/bin/rbt", line 9, in <module>

    load_entry_point('RBTools==0.5.2.dev', 'console_scripts', 'rbt')()

  File "/Users/gps/src/rbtools/rbtools/commands/main.py", line 113, in main

    command.run_from_argv([RB_MAIN] + args)

  File "/Users/gps/src/rbtools/rbtools/commands/init.py", line 157, in run_from_argv

    exit_code = self.main(*args) or 0

  File "/Users/gps/src/rbtools/rbtools/commands/diff.py", line 95, in main

    files=args)

  File "/Users/gps/src/rbtools/rbtools/utils/diffs.py", line 13, in get_diff

    repository_info)

  File "/Users/gps/src/rbtools/rbtools/clients/mercurial.py", line 388, in diff_between_revisions

    self._set_description(revision_range)

  File "/Users/gps/src/rbtools/rbtools/clients/mercurial.py", line 379, in _set_description

    if self.options.guess_description and not self.options.description:

AttributeError: Values instance has no attribute 'guess_description'
This patch makes the exceptions go away.
This is my first patch to rbtools. www.reviewboard.org/docs/codebase/dev/getting-started/ says "See our guidelines on Contributing Patches for more information" but there is no hyperlink, so I'm not sure if I'm doing this right. Apologies if I get anything wrong.