diff.c: emit_diff_symbol learns DIFF_SYMBOL_FILEPAIR_{PLUS, MINUS}

diff.c: emit_diff_symbol learns DIFF_SYMBOL_FILEPAIR_{PLUS, MINUS}

We have to use fprintf instead of emit_line, because we want to emit the
tab after the color. This is important for ancient versions of gnu patch
AFAICT, although we probably do not want to feed colored output to the
patch utility, such that it would not matter if the trailing tab is
colored. Keep the corner case as-is though.

Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>