buildbot/buildbot — pull request #1071 by tardyp (2014-02-19 14:39:32+00:00)
Title: db: add 1:N parentship between build and buildset

a build can be a parent of another build via trigger,
inherit force scheduler, and rebuild button.

This commit only add the data, and make the UT works again

subsequent commit will implement the logic

for early review only. its not finished yet