diffcore.h: be explicit about the signedness of is_binary

Bitfields need to specify their signedness explicitly or the compiler is
free to default as it sees fit.  With compilers that default 'unsigned'
(SUNWspro 12 seems to do this) the tri-state nature of is_binary
vanishes and all files are treated as binary.

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