Implement BazaarClient.parse_revision_spec and add tests.
This implements revision parsing for the BazaarClient. This is a little bit

deficient in that it can't automatically detect whether or not a parent branch

is required when using a revision range. I still haven't entirely figured out

how bazaar's bizarre branch mechanism works, but this at least reproduces what

we had until now, and includes parent diff generation.

Ran unit tests.