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.

Unit tests pass.
Ran rbt dff -X .. in a Bazaar repository and got the specified file was

not included.