use strbuf_addstr() instead of strbuf_addf() with "%s"

Call strbuf_addstr() for adding a simple string to a strbuf instead of
using the heavier strbuf_addf().  This is shorter and documents the
intent more clearly.

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