Ensure "existingfile" doesn't choke on "-".

mapper.go

@@ -555,6 +555,7 @@ func existingFileMapper(r *Registry) MapperFunc {
 		if err != nil {
 			return err
 		}
+		if path != "-" {
 			path = ExpandPath(path)
 			stat, err := os.Stat(path)
 			if err != nil {
@@ -563,6 +564,7 @@ func existingFileMapper(r *Registry) MapperFunc {
 			if stat.IsDir() {
 				return errors.Errorf("%q exists but is a directory", path)
 			}
+		}
 		target.SetString(path)
 		return nil
 	}