Fix stackoverflow in implicit FilesInfo JsonFormat

Fixes #61

* move it back into style
* use that in Tracked (uncomment some code)
* specify the style in the spec test
* must define absolute file.. (doesn't pass with relative, which is wrong imo)

internal/util-cache/src/main/scala/sbt/internal/util/FileInfo.scala

@@ -23,7 +23,6 @@ private final case class FileHashModified(file: File, hash: List[Byte], lastModi
 
 final case class FilesInfo[F <: FileInfo] private (files: Set[F])
 object FilesInfo {
-  implicit def format[F <: FileInfo]: JsonFormat[FilesInfo[F]] = implicitly
   def empty[F <: FileInfo]: FilesInfo[F] = FilesInfo(Set.empty[F])
 }
 
@@ -32,6 +31,7 @@ object FileInfo {
     type F <: FileInfo
 
     implicit def format: JsonFormat[F]
+    implicit def formats: JsonFormat[FilesInfo[F]] = project(_.files, (fs: Set[F]) => FilesInfo(fs))
 
     def apply(file: File): F
     def apply(files: Set[File]): FilesInfo[F] = FilesInfo(files map apply)

internal/util-cache/src/test/scala/FileInfoSpec.scala

@@ -4,11 +4,11 @@ import scala.json.ast.unsafe._
 import sjsonnew._, support.scalajson.unsafe._
 
 class FileInfoSpec extends UnitSpec {
-  val file = new java.io.File(".")
+  val file = new java.io.File(".").getAbsoluteFile
   val fileInfo: ModifiedFileInfo = FileModified(file, file.lastModified())
   val filesInfo = FilesInfo(Set(fileInfo))
 
-  it should "round trip" in assertRoundTrip(filesInfo)
+  it should "round trip" in assertRoundTrip(filesInfo)(FileInfo.lastModified.formats, FileInfo.lastModified.formats)
 
   def assertRoundTrip[A: JsonWriter: JsonReader](x: A) = {
     val jsonString: String = toJsonString(x)

internal/util-tracking/src/main/scala/sbt/internal/util/Tracked.scala

@@ -131,7 +131,7 @@ class Difference(val store: CacheStore, val style: FileInfo.Style, val defineCle
     }
   private def clearCache() = store.delete()
 
-  private def cachedFilesInfo = store.read(default = FilesInfo.empty[style.F]).files //(style.formats).files
+  private def cachedFilesInfo = store.read(default = FilesInfo.empty[style.F])(style.formats).files
   private def raw(fs: Set[style.F]): Set[File] = fs.map(_.file)
 
   def apply[T](files: Set[File])(f: ChangeReport[File] => T): T =
@@ -164,7 +164,7 @@ class Difference(val store: CacheStore, val style: FileInfo.Style, val defineCle
       val result = f(report)
       val info = if (filesAreOutputs) style(abs(extractFiles(result))) else currentFilesInfo
 
-      store.write(info)
+      store.write(info)(style.formats)
 
       result
     }