Git.pm: Use File::Temp->tempfile instead of ->new

Perl 5.8.0 ships with File::Temp 0.13, which does not have the new()
interface introduced in 0.14, as pointed out by Tom G. Christensen.

This modifies Git.pm to use the more established tempfile() interface
and updates 'git svn' to match.

Signed-off-by: Marcus Griep <marcus@griep.us>
Acked-by: Eric Wong <normalperson@yhbt.net>
Tested-by: Tom G. Christensen <tgc@statsbiblioteket.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>