diff-highlight: make perl strict and warnings fatal

These perl features can catch bugs, and we shouldn't be
violating any of the strict rules or creating any warnings,
so let's turn them on.

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

contrib/diff-highlight/diff-highlight

@@ -1,5 +1,8 @@
 #!/usr/bin/perl
 
+use warnings FATAL => 'all';
+use strict;
+
 # Highlight by reversing foreground and background. You could do
 # other things like bold or underline if you prefer.
 my $HIGHLIGHT   = "\x1b[7m";