Add link to corresponding issue in Scala issue tracker

compile/interface/src/main/scala/xsbt/Dependency.scala

@@ -150,6 +150,7 @@ final class Dependency(val global: CallbackGlobal) extends LocateClassFile
 				 * Some macros appear to contain themselves as original tree
 				 * In this case, we don't need to inspect the original tree because
 				 * we already inspected its expansion, which is equal.
+				 * See https://issues.scala-lang.org/browse/SI-8486
 				 */
 				case MacroExpansionOf(original) if original != tree =>
 					this.traverse(original)

compile/interface/src/main/scala/xsbt/ExtractUsedNames.scala

@@ -59,6 +59,7 @@ class ExtractUsedNames[GlobalType <: CallbackGlobal](val global: GlobalType) ext
 				// Some macros seem to have themselves registered as original tree.
 				// In this case, we only need to handle the children of the original tree,
 				// because we already handled the expanded tree.
+				// See https://issues.scala-lang.org/browse/SI-8486
 				if(original == node) original.children.foreach(handleTreeNode)
 				else original.foreach(handleTreeNode)
 			}