Fixes #1478 - Append new sLog when reloading in set command.

* Migrate weak reference into logger class so we can test clearing it.
* Ensure new state.log is propoagted into settings on `set` command.
* Fix set test so that it ensures the sLog is relatively stable when
  reloading on set command.
Add PR notes.
Merge pull request #1486 from sbt/fix/issue-1478

Fixes #1478 - Append new sLog when reloading in set command.
Fixes #1375 - Check for empty filenames.

Starting sbt in root isn't a great idea, but it shouldn't break.
Add notes so we have a merge conflict.

Conflicts:
	notes/0.13.6.md