use strbuf_addstr() for adding constant strings to a strbuf

use strbuf_addstr() for adding constant strings to a strbuf

Replace uses of strbuf_addf() for adding strings with more lightweight
strbuf_addstr() calls.

In http-push.c it becomes easier to see what's going on without having
to verfiy that the definition of PROPFIND_ALL_REQUEST doesn't contain
any format specifiers.

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