Finish tranisition for new revisions syntax.

This change does a handful of things:
* Updates the ClearCase implementation to use the new diff method, and changes
  it from using --tracking-branch to a `brtype:` prefix.
* Fixes up ClearCase documentation for the new syntax.
* Cleans up the `get_diff` method, now that there are no legacy consumers.
* Changes the `--revision-range` argument to just print an error.
* Changes the help for the options to say `[revisions]` instead of
  `[changenum]`.
* Updates the docs to remove --revision-range
* Updates the docs to list -I/--include

Testing done:
* Ran unit tests.
* Built docs and verified that they looked right.

Reviewed at https://reviews.reviewboard.org/r/5264/