In ActivetState Perl, exec does not wait for the started program. This
breaks difftool tests and may cause unexpected behaviour: git difftool
has returned, but the rest of code (diff and possibly the interactive
program are still running in the background.
Acked-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>