diff-highlight: do not split multibyte characters

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>