effetflag: detect other meta (extra) changes Check other changeset meta other than branch and *_source and if there is a change, set the METACHANGED effect flag. Add a test for this scenario with a topic change.

obsdiscovery: add an option to disable all obsdiscovery On large repository, computing the obshashtree can we as slow as resending everything. We give the option for people to opt out.

effectflag: better diff detection Improve the diff detection algorithm to have less false positive like in the case of a rebase that move the hunk in the file but the hunk stays the same.