Add -X support for the CVS SCMClient

The CVS SCMClient can now exclude files from `diff` and `post` output using the
`rbtools.utils.diffs.filter_diff` method.

Testing Done:
Running `rbt diff -X f` in a file with more than just a file `f` changed
returns the diff for all files except for `f`.

Reviewed at https://reviews.reviewboard.org/r/6358/