diff.c: add set_sign to emit_line_0

diff.c: add set_sign to emit_line_0

Split the meaning of the `set` parameter that is passed to
emit_line_0()` to separate between the color of the "sign" (i.e.
the diff marker '+', '-' or ' ' that is passed in as the `first`
parameter) and the color of the rest of the line.

This changes the meaning of the `set` parameter to no longer refer
to the color of the diff marker, but instead to refer to the color
of the rest of the line. A value of `NULL` indicates that the rest
of the line wants to be colored the same as the diff marker.

Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>