fix System::Command to run in environments that mess with STDIN/STDOUT/STDERR

This patch was contributed by Todd Rinaldo (TODDR) to Git::Repository in 2010,
and got lost when System::Command was forked out of Git::Repository.
(commit 443a94018f5577575d548b8db4e4c313f5c70377 in the Git-Repository repository)

IPC::Open3 has issues when run under FCGI or Plack,
and this patch fixes the issue.
Changes for version 1.102