[devel at bb.net] Mergeable Buildbot pull requests

[dustin at buildbot.net]

Mergeable pull requests:

https://github.com/buildbot/buildbot/pull/2016 - fix problem where parent_changeids might be corrupted
https://github.com/buildbot/buildbot/pull/2012 - Move changes in private API during worker transition to release notes
https://github.com/buildbot/buildbot/pull/2011 - check that ShellCommand command in set in constructor