Fix cross multi command performance
Fix cross multi command performance

In 53788ba3561a4e79bc9c36c329f43540ad48814e, 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.

sbt/sbt,mdedetrich/sbt,sbt/sbt,xuwei-k/xsbt,mdedetrich/sbt,sbt/sbt,mdedetrich/sbt,xuwei-k/xsbt,sbt/sbt,xuwei-k/xsbt,sbt/sbt,sbt/sbt,xuwei-k/xsbt,mdedetrich/sbt,xuwei-k/xsbt,sbt/sbt,mdedetrich/sbt,mdedetrich/sbt,mdedetrich/sbt