Breezy is a modernization of Bazaar, designed to work with Python 3 and

to iterate on Bazaar's feature set. It's mostly a drop-in replacement,

but has a couple changes which matter to us:


It supports Git, and that triggered some bad matching logic (fixed

   in https://reviews.reviewboard.org/r/11686/).


Its generated diffs default to having old/ and new/ prefixes

   before filenames on the header lines, which Review Board doesn't

   expect.


This change implements support for Breezy by checking both bzr and

brz (the official Breezy command line tool name) and setting the

proper command to run for all interactions, along with setting a flag to

trigger different behaviors for diff building (to address change #2

above).
With this, we should have support for both Bazaar 2.7 (the final

version) and Breezy 3.2 (the latest version).

Unit tests pass on both Python 2 and 3 for Bazaar and Breezy.