diff: use emit_line_0 once per line

diff: use emit_line_0 once per line

All lines that use emit_line_0 multiple times per line, are combined
into a single call to emit_line_0, making use of the 'set' argument.

We gain a little efficiency here, as we can omit emission of color and
accompanying reset if 'len == 0'.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>