Add scripted (functional test) for #2324.

Add a test that shows the scenario described in #2324 is handled correctly
now.

sbt/src/sbt-test/compiler-project/private-public-both-modified/build.sbt

@@ -0,0 +1,25 @@
+/* Performs checks related to compilations:
+ *  a) checks in which compilation given set of files was recompiled
+ *  b) checks overall number of compilations performed
+ */
+TaskKey[Unit]("check-compilations") := {
+  val analysis = (compile in Compile).value
+  val srcDir = (scalaSource in Compile).value
+  def relative(f: java.io.File): java.io.File =  f.relativeTo(srcDir) getOrElse f
+  val allCompilations = analysis.compilations.allCompilations
+  val recompiledFiles: Seq[Set[java.io.File]] = allCompilations map { c =>
+    val recompiledFiles = analysis.apis.internal.collect {
+      case (file, api) if api.compilation.startTime == c.startTime => relative(file)
+    }
+    recompiledFiles.toSet
+  }
+  def recompiledFilesInIteration(iteration: Int, fileNames: Set[String]) = {
+    val files = fileNames.map(new java.io.File(_))
+    assert(recompiledFiles(iteration) == files, "%s != %s".format(recompiledFiles(iteration), files))
+  }
+  assert(allCompilations.size == 2)
+  // B.scala is just compiled at the beginning
+  recompiledFilesInIteration(0, Set("B.scala"))
+  // A.scala is changed and recompiled
+  recompiledFilesInIteration(1, Set("A.scala"))
+}

sbt/src/sbt-test/compiler-project/private-public-both-modified/changes/A1.scala

@@ -0,0 +1,5 @@
+object A {
+  private def foo: String = "1"
+  def bar: String = "29"
+  def xyz: Int = 101
+}

sbt/src/sbt-test/compiler-project/private-public-both-modified/src/main/scala/A.scala

@@ -0,0 +1,5 @@
+object A {
+  private def foo: Int = 1
+  def bar: Int = 29
+  def xyz: Int = 101
+}

sbt/src/sbt-test/compiler-project/private-public-both-modified/src/main/scala/B.scala

@@ -0,0 +1,4 @@
+class B {
+  private def foo: Int = 1
+  def baz(): Unit = println(foo, A.xyz)
+}

sbt/src/sbt-test/compiler-project/private-public-both-modified/test

@@ -0,0 +1,9 @@
+# Test for https://github.com/sbt/sbt/issues/2324
+# introduces first compile iteration
+> compile
+# change signature of a private `A.foo` method
+$ copy-file changes/A1.scala src/main/scala/A.scala
+# it should recompile just A.scala because changes to `A.foo` can't affect B
+> compile
+# check if there are only two compile iterations performed
+> check-compilations