t5000: tar portability fix

t5000: tar portability fix

The output of 'tar tv' varies from system to system. In
particular, the t5000 was expecting to parse the date from
something like:

  -rw-rw-r-- root/root         0 2008-05-13 04:27 file

but FreeBSD's tar produces this:

  -rw-rw-r--  0 root   root        0 May 13 04:27 file

Instead of relying on tar's output, let's just extract the
file using tar and stat the result using perl.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>