Do not append default watch sources with ++=
Do not append default watch sources with ++=

Using ++= prevents these from being overridden by the user without doing
some nasty runtime manipulations.

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