Don't try to stamp files that don't exist
Don't try to stamp files that don't exist

This was causing slowdowns in windows.

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