Fix cross multi command performance

In 53788ba356, I changed the cross multi
parser to issue all of the commands sequentially. This caused a
performance regression for many use cases:
https://github.com/sbt/sbt/issues/5321. This commit restores the old
behavior of `+` if the command to run has no arguments.

main/src/main/scala/sbt/Cross.scala

@@ -194,7 +194,13 @@ object Cross {
           .sortBy(_._1)
         commandsByVersion.flatMap {
           case (v, commands) =>
-            Seq(s"$SwitchCommand $verbose $v!") ++ commands
+            commands match {
+              case Seq(c) => Seq(s"$SwitchCommand $verbose $v! $c")
+              case Seq()  => Nil // should be unreachable
+              case multi if fullArgs.isEmpty =>
+                Seq(s"$SwitchCommand $verbose $v! all ${multi.mkString(" ")}")
+              case multi => Seq(s"$SwitchCommand $verbose $v!") ++ multi
+            }
         }
     }
     allCommands.toList ::: CrossRestoreSessionCommand :: captureCurrentSession(state, extracted)