pretty: avoid adding reset for %C(auto) if output is empty

pretty: avoid adding reset for %C(auto) if output is empty

We emit an escape sequence for resetting color and attribute for
%C(auto) to make sure automatic coloring is displayed as intended.
Stop doing that if the output strbuf is empty, i.e. when %C(auto)
appears at the start of the format string, because then there is no
need for a reset and we save a few bytes in the output.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>