Fix race condition on Windows by consolidating two file writes

src/main/scala/scoverage/Invoker.scala

@@ -5,10 +5,19 @@ import java.io.FileWriter
 /** @author Stephen Samuel */
 object Invoker {
 
+  /**
+   * We record that the given id has been invoked by appending its id to the coverage
+   * data file.
+   * This will happen concurrently on as many threads as the application is using,
+   * but appending small amounts of data to a file is atomic on both POSIX and Windows
+   * if it is a single write of a small enough string.
+   *
+   * @see http://stackoverflow.com/questions/1154446/is-file-append-atomic-in-unix
+   * @see http://stackoverflow.com/questions/3032482/is-appending-to-a-file-atomic-with-windows-ntfs
+   */
   def invoked(id: Int, path: String) = {
     val writer = new FileWriter(path, true)
-    writer.append(id.toString)
-    writer.append(';')
+    writer.append(id.toString + ';')
     writer.close()
   }
 }