diff.c: pass sign_index to emit_line_ws_markup

Instead of passing the sign directly to emit_line_ws_markup, pass only the
index to lookup the sign in diff_options->output_indicators.

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