convert: add a config knob for not saving the bzr revision Now that the timestamp is fixed, the log of the Mercurial repo is still unstable with --debug, because the bzr commit ID keeps changing. Both hg and git implement `convert.*.saverev`, so do the same here.

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

synthrepo: pass a diffopts object to context.diff

webutil: pass a diffopts object to context.diff

revset: pass an explicit `diffopts` objet to context.diff