buildrequest collapse

 - TODO:
   - update master/docs/manual/customization.rst

First iteration of the project.
I plan to update customization.rst.
Ok. I understand your point. It's a modification of the current implementation in buildbot.
If data.updates.completeBuildRequests have the responsability of completing the parent buildset, then 2 other functions must be patched:
 - BuildRequest::cancelBuildRequest.
 - Builder::_notify_completions

Indeed, these 2 functions call data.updates.completeBuildRequests and then call data.updates.maybeBuildsetComplete.