Use test-chmtime -v instead of perl in t5000 to get mtime of a file

The test was broken on admittedly broken combination of Windows, Cygwin,
and ActiveState Perl.

Signed-off-by: Alex Riesen <ariesen@harmanbecker.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5000-tar-tree.sh

@@ -90,7 +90,7 @@ test_expect_success \
     'validate file modification time' \
     'mkdir extract &&
      "$TAR" xf b.tar -C extract a/a &&
-     perl -e '\''print((stat("extract/a/a"))[9], "\n")'\'' >b.mtime &&
+     test-chmtime -v +0 extract/a/a |cut -f 1 >b.mtime &&
      echo "1117231200" >expected.mtime &&
      diff expected.mtime b.mtime'