Add support for -X to the Bazaar SCM Client.

Bazaar now supports excluding files from diffs and posts via
`rbtools.utils.diffs.filter_diff`, which looks through the output of bzr diff
for lines indicating the start of a file's diff. If the file specified matches
any pattern, that file's diff is excluded from the resulting diff or post.

Add a test for checking the result of the BazaarClient's diff with file
exclusion.

Testing Done:
Unit tests pass.

Ran `rbt dff -X ..` in a Bazaar repository and got the specified file was not
included.

Reviewed at https://reviews.reviewboard.org/r/6349/