update the extended_header.tar test file

There was a bug in the original test file which is now properly caught
by the Go tar package. Update the file with a current version of git,
and re-enable the test.

decompress_tar_test.go

@@ -9,18 +9,14 @@ import (
 func TestTar(t *testing.T) {
 	mtime := time.Unix(0, 0)
 	cases := []TestDecompressCase{
-		/*
-			Disabled for now, this was broken in Go 1.10 and doesn't parse at
-			all anymore. Issue open here: https://github.com/golang/go/issues/28843
-			{
-				"extended_header.tar",
-				true,
-				false,
-				[]string{"directory/", "directory/a", "directory/b"},
-				"",
-				nil,
-			},
-		*/
+		{
+			"extended_header.tar",
+			true,
+			false,
+			[]string{"directory/", "directory/a", "directory/b"},
+			"",
+			nil,
+		},
 		{
 			"implied_dir.tar",
 			true,