Undone the revert on the optimization, and fixed setAll()

The optimization, and therefore the change in the behavior
of Relation, is now needed by the class Logic, and cannot
be reverted.

This patch (written by Josh) therefore changes the
implementation of setAll() so that _1s is no longer used.

main/src/main/scala/sbt/SettingCompletions.scala

@@ -27,7 +27,7 @@ private[sbt] object SettingCompletions {
     {
       import extracted._
       val r = relation(extracted.structure, true)
-      val allDefs = r._1s.toSeq
+      val allDefs = Def.flattenLocals(Def.compiled(extracted.structure.settings, true)(structure.delegates, structure.scopeLocal, implicitly[Show[ScopedKey[_]]])).map(_._1)
       val projectScope = Load.projectScope(currentRef)
       def resolve(s: Setting[_]): Seq[Setting[_]] = Load.transformSettings(projectScope, currentRef.build, rootProject, s :: Nil)
       def rescope[T](setting: Setting[T]): Seq[Setting[_]] =
@@ -353,4 +353,4 @@ private[sbt] object SettingCompletions {
     classOf[Long],
     classOf[String]
   )
-}
+}

util/relation/src/main/scala/sbt/Relation.scala

@@ -129,7 +129,7 @@ private final class MRelation[A, B](fwd: Map[A, Set[B]], rev: Map[B, Set[A]]) ex
 
   def +(pair: (A, B)) = this + (pair._1, Set(pair._2))
   def +(from: A, to: B) = this + (from, to :: Nil)
-  def +(from: A, to: Traversable[B]) =
+  def +(from: A, to: Traversable[B]) = if (to.isEmpty) this else
     new MRelation(add(fwd, from, to), (rev /: to) { (map, t) => add(map, t, from :: Nil) })
 
   def ++(rs: Traversable[(A, B)]) = ((this: Relation[A, B]) /: rs) { _ + _ }