Backport the nts() function from the trunk. This fixes problems with
the xstar format that puts extra fields inside the space that POSIX
has reserved for the prefix field.
diff --git a/Lib/test/test_tarfile.py b/Lib/test/test_tarfile.py
index dc28038..a9797ae 100644
--- a/Lib/test/test_tarfile.py
+++ b/Lib/test/test_tarfile.py
@@ -26,7 +26,7 @@
 testtar = path("testtar.tar")
 tempdir = os.path.join(tempfile.gettempdir(), "testtar" + os.extsep + "dir")
 tempname = test_support.TESTFN
-membercount = 12
+membercount = 13
 
 def tarname(comp=""):
     if not comp:
@@ -225,6 +225,12 @@
             self.assertEqual(tarinfo.mtime, os.path.getmtime(path))
         tar.close()
 
+    def test_star(self):
+        try:
+            self.tar.getmember("7-STAR")
+        except KeyError:
+            self.fail("finding 7-STAR member failed (mangled prefix?)")
+
 
 class ReadStreamTest(ReadTest):
     sep = "|"