Wait for git diff to finish in git difftool

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>

git-difftool.perl

@@ -82,4 +82,5 @@ sub generate_command
 }
 
 setup_environment();
-exec(generate_command());
+my $rc = system(generate_command());
+exit($rc | ($rc >> 8));