Issue #13158: Fix decoding and encoding of base-256 number fields in tarfile.

The nti() function that converts a number field from a tar header to a number
failed to decode GNU tar specific base-256 fields. I also added support for
decoding and encoding negative base-256 number fields.
diff --git a/Misc/NEWS b/Misc/NEWS
index ecff9b7..77bc91f 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -43,6 +43,9 @@
 Library
 -------
 
+- Issue #13158: Fix decoding and encoding of GNU tar specific base-256 number
+  fields in tarfile.
+
 - Issue #13025: mimetypes is now reading MIME types using the UTF-8 encoding,
   instead of the locale encoding.