diff-highlight: do not split multibyte characters

When the input is UTF-8 and Perl is operating on bytes instead of characters, a diff that changes one multibyte character to another that shares an initial byte sequence will result in a broken diff display as the common byte sequence prefix will be separated from the rest of the bytes in the multibyte character. For example, if a single line contains only the unicode character U+C9C4 (encoded as UTF-8 0xEC, 0xA7, 0x84) and that line is then changed to the unicode character U+C9C0 (encoded as UTF-8 0xEC, 0xA7, 0x80), when operating on bytes diff-highlight will show only the single byte change from 0x84 to 0x80 thus creating invalid UTF-8 and a broken diff display. Fix this by putting Perl into character mode when splitting the line and then back into byte mode after the split is finished. The utf8::xxx functions require Perl 5.8 so we require that as well. Also, since we are mucking with code in the split_line function, we change a '*' quantifier to a '+' quantifier when matching the $COLOR expression which has the side effect of speeding everything up while eliminating useless '' elements in the returned array. Reported-by: Yi EungJun <semtlenori@gmail.com> Signed-off-by: Kyle J. McKay <mackyle@gmail.com> Acked-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>