Use project watchStartMessage for multi commands

It didn't make sense to aggregate the watch start command if it was
defined in multiple sources so we previously just fell back to the
default message if multiple commands were being run. This, however,
meant that if you ran, say, ~compile in an aggregate project, it wasn't
possible to customize the start message. There was a message in the
sbt gitter channel where someone found the new message too verbose and
wanted to print something shorter and I realized that this was an
unfortunate restriction. Instead of giving up, we can just use the
project's watchStartMessage as a default. If the watchStartMessage
setting is unset for some reason, we can fall back to the default.

I validated this change manually in the swoval project, which has an
aggregate root project, by running
set ThisBuild / watchStartMessage := { (_, _, _) => None }
and indeed nothing was printed after each task evaluation in '~compile'.

main/src/main/scala/sbt/internal/Continuous.scala

@@ -469,7 +469,8 @@ private[sbt] object Continuous extends DeprecatedContinuous {
     val project = extracted.currentRef
     val logger = setLevel(rawLogger, configs.map(_.watchSettings.logLevel).min, state)
     val beforeCommand = () => configs.foreach(_.watchSettings.beforeCommand())
-    val onStart: () => Watch.Action = getOnStart(project, commands, configs, rawLogger, count)
+    val onStart: () => Watch.Action =
+      getOnStart(project, commands, configs, rawLogger, count, extracted)
     val nextInputEvent: () => Watch.Action = parseInputEvents(configs, state, inputStream, logger)
     val (nextFileEvent, cleanupFileMonitor): (() => Option[(Watch.Event, Watch.Action)], () => Unit) =
       getFileEvents(configs, rawLogger, state, count, commands, fileStampCache)
@@ -501,7 +502,8 @@ private[sbt] object Continuous extends DeprecatedContinuous {
       commands: Seq[String],
       configs: Seq[Config],
       logger: Logger,
-      count: AtomicInteger
+      count: AtomicInteger,
+      extracted: Extracted,
   ): () => Watch.Action = {
     val f: () => Seq[Watch.Action] = () => {
       configs.map { params =>
@@ -522,8 +524,11 @@ private[sbt] object Continuous extends DeprecatedContinuous {
     () => {
       val res = f().min
       // Print the default watch message if there are multiple tasks
-      if (configs.size > 1)
-        Watch.defaultStartWatch(count.get(), project, commands).foreach(logger.info(_))
+      if (configs.size > 1) {
+        val onStartWatch =
+          extracted.getOpt(watchStartMessage in project).getOrElse(Watch.defaultStartWatch)
+        onStartWatch(count.get(), project, commands).foreach(logger.info(_))
+      }
       res
     }
   }