git-cvsexportcommit: support Perl before 5.10.1

The change in 6e9c4d4 ("git-cvsexportcommit: port to SHA-256",
2020-06-22) added the use of a temporary directory for the index.
However, the form we used doesn't work in versions of Perl before
5.10.1.  For example, version 5.10.0 contains a version of File::Temp
from 2007 that doesn't contain "newdir".

In order to make the code work with 5.8.8, which we support, let's
change to use the static method "tempdir" with the argument "CLEANUP",
which provides the same behavior.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-cvsexportcommit.perl

@@ -22,7 +22,7 @@
 my $repo = Git->repository();
 $opt_w = $repo->config('cvsexportcommit.cvsdir') unless defined $opt_w;
 
-my $tmpdir = File::Temp->newdir;
+my $tmpdir = File::Temp::tempdir(CLEANUP => 1);
 my $hash_algo = $repo->config('extensions.objectformat') || 'sha1';
 my $hexsz = $hash_algo eq 'sha256' ? 64 : 40;