graph: use strbuf_addchars() to add spaces

graph: use strbuf_addchars() to add spaces

strbuf_addf() can be used to add a specific number of space characters
by using the format "%*s" with an empty string and specifying the
desired width.  Use strbuf_addchars() instead as it's shorter, makes the
intent clearer and is a bit more efficient.

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