physical File.eq in conformance to Isabelle/ML;

--- a/src/Pure/General/file.scala	Thu Sep 27 14:52:50 2012 +0200
+++ b/src/Pure/General/file.scala	Thu Sep 27 15:04:26 2012 +0200
@@ -105,7 +105,7 @@
   /* copy */
 
   def eq(file1: JFile, file2: JFile): Boolean =
-    file1.getCanonicalPath == file2.getCanonicalPath  // FIXME prefer java.nio.file.Files.isSameFile of Java 1.7
+    java.nio.file.Files.isSameFile(file1.toPath, file2.toPath)
 
   def copy(src: JFile, dst: JFile)
   {