Issue scala#25 use filtered check for git diff

Unlike normal diff, the git diff feature wasn't using the filtered
check file under `--show-diff`.

Now the sample displays just the bad line.

```
!! 1 - neg/t7494-no-options                      [output differs]
% diff /home/apm/projects/snytt/test/files/neg/t7494-no-options-neg.log /home/apm/projects/snytt/test/files/neg/t7494-no-options.check
@@ -1,7 +1,7 @@
 error: Error: ploogin takes no options
     phase name  id  description
     ----------  --  -----------
-        parser   1  parse source into ASTs, perform simple desugaring
+        parser   0  parse source into ASTs, perform simple desugaring
          namer   2  resolve names, attach symbols to named trees
 packageobjects   3  load package objects
          typer   4  the meat and potatoes: type the trees
```

src/partest/scala/tools/partest/nest/FileManager.scala

@@ -10,15 +10,9 @@ package nest
 
 import java.io.{
   File,
-  FilenameFilter,
   IOException,
-  StringWriter,
-  FileInputStream,
-  FileOutputStream,
-  BufferedReader,
-  FileReader,
-  PrintWriter,
-  FileWriter
+  OutputStreamWriter,
+  FileOutputStream
 }
 import java.net.URI
 import scala.reflect.io.AbstractFile
@@ -98,6 +92,19 @@ object FileManager {
     if (diff.getDeltas.isEmpty) ""
     else difflib.DiffUtils.generateUnifiedDiff(originalName, revisedName, original.asJava, diff, 1).asScala.mkString("\n")
   }
+
+  def withTempFile[A](outFile: File, fileBase: String, lines: Seq[String])(body: File => A): A = {
+    val prefix = s"tmp-$fileBase"
+    val suffix = ".check"
+    val f = File.createTempFile(prefix, suffix, outFile)
+    try {
+      import scala.reflect.io.{ File => Feil }
+      Feil(f).writeAll(lines map (line => f"$line%n"): _*)
+      body(f)
+    } finally {
+      f.delete()
+    }
+  }
 }
 
 class FileManager(val testClassLoader: URLClassLoader) {

src/partest/scala/tools/partest/nest/Runner.scala

@@ -18,14 +18,13 @@ import scala.reflect.internal.util.ScalaClassLoader
 import scala.sys.process.{ Process, ProcessLogger }
 import scala.tools.nsc.Properties.{ envOrNone, isWin, jdkHome, javaHome, propOrEmpty, setProp, versionMsg, javaVmName, javaVmVersion, javaVmInfo }
 import scala.tools.nsc.{ Settings, CompilerCommand, Global }
-import scala.tools.nsc.io.{ AbstractFile }
 import scala.tools.nsc.reporters.ConsoleReporter
 import scala.tools.nsc.util.{ Exceptional, stackTraceString }
 import scala.util.{ Try, Success, Failure }
 import ClassPath.{ join, split }
 import TestState.{ Pass, Fail, Crash, Uninitialized, Updated }
 
-import FileManager.{compareFiles, compareContents, joinPaths}
+import FileManager.{ compareFiles, compareContents, joinPaths, withTempFile }
 
 class TestTranscript {
   import NestUI.color._
@@ -375,13 +374,13 @@ class Runner(val testFile: File, val suiteRunner: SuiteRunner) {
         genUpdated()
       case Some(false) =>
         // Get a word-highlighted diff from git if we can find it
-        val bestDiff = if (updating.isEmpty) "" else {
-          if (checkFile.canRead)
-            gitDiff(logFile, checkFile) getOrElse {
-              s"diff $logFile $checkFile\n$diff"
+        val bestDiff =
+          if (updating.isEmpty) ""
+          else if (checkFile.canRead)
+            withTempFile(outFile, fileBase, filteredCheck) { f =>
+              gitDiff(logFile, f) getOrElse f"diff $logFile $checkFile%n$diff"
             }
           else diff
-        }
         _transcript append bestDiff
         genFail("output differs")
         // TestState.fail("output differs", "output differs",
@@ -606,7 +605,7 @@ class Runner(val testFile: File, val suiteRunner: SuiteRunner) {
     val prompt = "\nnsc> "
     val (swr, wr) = newTestWriters()
 
-    NestUI.verbose(this+" running test "+fileBase)
+    NestUI.verbose(s"$this running test $fileBase")
     val dir = parentFile
     val resFile = new File(dir, fileBase + ".res")