proper toString for Content_XML, which is not covered by trait Content;

--- a/src/Pure/General/file.scala	Fri Aug 12 15:48:47 2022 +0200
+++ b/src/Pure/General/file.scala	Fri Aug 12 15:57:22 2022 +0200
@@ -330,6 +330,8 @@
   }
 
   final class Content_XML private[File](val path: Path, val content: XML.Body) {
+    override def toString: String = path.toString
+
     def output(out: XML.Body => String): Content_String =
       new Content_String(path, out(content))
   }