Fix watch command parser

I discovered that when I ran multi-commands with '~' that if there was a
space between the ';' and the command, then the parsing of the command
would fail and the watch would abort. To fix this, I refactor
Watched.watch to use the multi command parser and, if that parser fails,
we fallback on a single command.

main-command/src/main/scala/sbt/Watched.scala

@@ -21,7 +21,7 @@ import sbt.internal.LegacyWatched
 import sbt.internal.inc.Stamper
 import sbt.internal.io.{ EventMonitor, Source, WatchState }
 import sbt.internal.util.Types.const
-import sbt.internal.util.complete.DefaultParsers
+import sbt.internal.util.complete.{ DefaultParsers, Parser }
 import sbt.internal.util.{ AttributeKey, JLine }
 import sbt.io.FileEventMonitor.{ Creation, Deletion, Event, Update }
 import sbt.io._
@@ -279,9 +279,9 @@ object Watched {
       onFailure = Some(Exec(failureCommandName, None)),
       definedCommands = s0.definedCommands :+ onFail
     )
-    val commands = command.split(";") match {
-      case Array("", rest @ _*) => rest
-      case Array(cmd)           => Seq(cmd)
+    val commands = Parser.parse(command, BasicCommands.multiParserImpl(Some(s))) match {
+      case Left(_)  => command :: Nil
+      case Right(c) => c
     }
     val parser = Command.combine(s.definedCommands)(s)
     val tasks = commands.foldLeft(Nil: Seq[Either[String, () => Either[Exception, Boolean]]]) {

sbt/src/sbt-test/watch/watch-parser/build.sbt

@@ -0,0 +1,13 @@
+import Build._
+
+organization := "sbt"
+
+name := "scripted-watch-parser"
+
+setStringValue := setStringValueImpl.evaluated
+
+checkStringValue := checkStringValueImpl.evaluated
+
+watchSources += file("string.txt")
+
+watchOnEvent := { _ => Watched.CancelWatch }

sbt/src/sbt-test/watch/watch-parser/project/Build.scala

@@ -0,0 +1,16 @@
+import sbt._
+
+object Build {
+  private[this] var string: String = ""
+  private[this] val stringFile = file("string.txt")
+  val setStringValue = inputKey[Unit]("set a global string to a value")
+  val checkStringValue = inputKey[Unit]("check the value of a global")
+  def setStringValueImpl: Def.Initialize[InputTask[Unit]] = Def.inputTask {
+    string = Def.spaceDelimited().parsed.mkString(" ").trim
+    IO.write(stringFile, string)
+  }
+  def checkStringValueImpl: Def.Initialize[InputTask[Unit]] = Def.inputTask {
+    assert(string == Def.spaceDelimited().parsed.mkString(" ").trim)
+    assert(IO.read(stringFile) == string)
+  }
+}

sbt/src/sbt-test/watch/watch-parser/test

@@ -0,0 +1,18 @@
+> ~; setStringValue foo; setStringValue bar
+
+> checkStringValue bar
+
+> ~;setStringValue foo;setStringValue bar; checkStringValue bar
+
+> ~;   setStringValue foo;setStringValue bar; checkStringValue bar
+
+> ~; setStringValue foo; setStringValue bar; checkStringValue bar
+
+> ~ setStringValue foo
+
+> checkStringValue foo
+
+# All of the other tests have involved input tasks, so include commands with regular tasks as well.
+> ~; compile; setStringValue baz; checkStringValue baz
+# No trailing semicolons are allowed
+-> ~; compile; setStringValue baz; checkStringValue baz;