Make scripted watch/on-changes more robust

I noticed that sometimes this test hangs in ci. I think it may be
because sometimes the event for A got handled while the cache entry for
B.scala was invalidated. Because the test runs ~compile, the next run of
compile would then reset the cache entry for B.scala. The next run of
watchOnIteration invalidtes B.scala again, but this time to the same
hash, so it doesn't actually trigger a file input event. By changing the
content of B.scala every time, we ensure that it should still cause a
file event.

I haven't confirmed that this will fix the sporadic hangs because I
can't reproduce on my machine, but this change won't hurt.

sbt/src/sbt-test/watch/on-change/build.sbt

@@ -1,8 +1,10 @@
 import java.nio.file._
+import java.nio.file.attribute.FileTime
+
 import sbt.nio.Keys._
 import sbt.nio._
+
 import scala.concurrent.duration._
-import StandardCopyOption.{ REPLACE_EXISTING => replace }
 
 watchTriggeredMessage := { (i, path: Path, c) =>
   val prev = watchTriggeredMessage.value
@@ -16,14 +18,15 @@ watchOnIteration := { i: Int =>
   val src =
     base.resolve("src").resolve("main").resolve("scala").resolve("sbt").resolve("test")
   val changes = base.resolve("changes")
-  Files.copy(changes.resolve("C.scala"), src.resolve("C.scala"), replace)
-  if (i < 5) {
+  def copy(fileName: String): Unit = {
     val content =
-      new String(Files.readAllBytes(changes.resolve("A.scala"))) + "\n" + ("//" * i)
-    Files.write(src.resolve("A.scala"), content.getBytes)
-  } else {
-    Files.copy(changes.resolve("B.scala"), src.resolve("B.scala"), replace)
+      new String(Files.readAllBytes(changes.resolve(fileName))) + "\n" + ("//" * i)
+    Files.write(src.resolve(fileName), content.getBytes)
   }
+  val c = src.resolve("C.scala")
+  Files.setLastModifiedTime(c, FileTime.fromMillis(Files.getLastModifiedTime(c).toMillis + 1111))
+  if (i < 5) copy("A.scala")
+  else copy("B.scala")
   println(s"Waiting for changes...")
   Watch.Ignore
 }