test-bzr: use a fixed commit time This will help stabilize the hashes when converting to a Mercurial repo.

diff: use `context.diff` to produce diff We want to make sure `context.diff` if full featured. Using it in the main diff code path is a good way to do so. The end goal is to be able to easily compute diff between in-memory context.

context: no longer accept diff options as dictionnary Since we already broke the API earlier in this stack, there are no point to introducing a new deprecation warning.

obsutil: pass a diffopts object to context.diff