use strbuf_addstr() for adding strings to strbufs

use strbuf_addstr() for adding strings to strbufs

Use strbuf_addstr() instead of strbuf_addf() for adding strings.  That's
simpler and makes the intent clearer.

Patch generated by Coccinelle and contrib/coccinelle/strbuf.cocci;
adjusted indentation in refs/packed-backend.c manually.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>