Rename API resource object "fields" to "_fields"

The internal fields dictionary should have been made "prviate" when
refactoring the API client to pass out actual resource objects instead
of wrapping them. This change corrects that mistake.

Reviewed at https://reviews.reviewboard.org/r/5137/
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.

Testing done: Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/5142/