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.

main/src/main/scala/sbt/LogManager.scala

@@ -105,8 +105,8 @@ object LogManager {
   // construct a Logger that delegates to the global logger, but only holds a weak reference
   //  this is an approximation to the ideal that would invalidate the delegate after loading completes
   private[this] def globalWrapper(s: State): Logger = {
-    val ref = new java.lang.ref.WeakReference(s.globalLogging.full)
     new Logger {
+      private[this] val ref = new java.lang.ref.WeakReference(s.globalLogging.full)
       private[this] def slog: Logger = Option(ref.get) getOrElse sys.error("Settings logger used after project was loaded.")
 
       override val ansiCodesSupported = slog.ansiCodesSupported

main/src/main/scala/sbt/Main.scala

@@ -227,7 +227,10 @@ object BuiltinCommands {
   def reapply(newSession: SessionSettings, structure: BuildStructure, s: State): State =
     {
       s.log.info("Reapplying settings...")
-      val newStructure = Load.reapply(newSession.mergeSettings, structure)(Project.showContextKey(newSession, structure))
+      // Here, for correct behavior, we also need to re-inject a settings logger, as we'll be re-evaluating settings.
+      val loggerInject = LogManager.settingsLogger(s)
+      val withLogger = newSession.appendRaw(loggerInject :: Nil)
+      val newStructure = Load.reapply(withLogger.mergeSettings, structure)(Project.showContextKey(newSession, structure))
       Project.setProject(newSession, newStructure, s)
     }
   def set = Command(SetCommand, setBrief, setDetailed)(setParser) {