Add toString for DirectCredentials

To make 'show credentials' more useful

ivy/src/main/scala/sbt/librarymanagement/ivy/Credentials.scala

@@ -76,11 +76,13 @@ object Credentials {
 
 sealed trait Credentials
 final class FileCredentials(val path: File) extends Credentials {
-  override def toString = "FileCredentials('" + path + "')"
+  override def toString = s"""FileCredentials("$path")"""
 }
 final class DirectCredentials(
     val realm: String,
     val host: String,
     val userName: String,
     val passwd: String
-) extends Credentials
+) extends Credentials {
+  override def toString = s"""DirectCredentials("$realm", "$host", "$userName", ****)"""
+}