proper init of non-existing file;

--- a/src/Pure/General/file.scala	Thu Mar 03 19:50:41 2022 +0100
+++ b/src/Pure/General/file.scala	Thu Mar 03 19:51:00 2022 +0100
@@ -247,7 +247,7 @@
 
   def change(path: Path, init: Boolean = false)(f: String => String): Unit =
   {
-    if (init) append(path, "")
+    if (!path.is_file && init) write(path, "")
     val x = read(path)
     val y = f(x)
     if (x != y) write(path, y)