sbt/sbt — pull request #1489 by jsuereth (2014-08-01 20:53:06+00:00)
Title: Fixes #856 - Store ++ scala version changes on session.

* Ensure the ++ command stores its changes on the sbt session.
* Make sure `session clear-all` will clear out ++ changes
* Validate that `set` command doesn't undo `++` changes

Note: There is some autogenerated Setting[_] delegate optimisation
work that could be done in the future.

Review by @eed3si9n cc @paulp