strbuf.cocci: suggest strbuf_addbuf() to add one strbuf to an other

The best way to add one strbuf to an other is via: strbuf_addbuf(&sb, &sb2); This is a bit more idiomatic and efficient than: strbuf_addstr(&sb, sb2.buf); because the size of the second strbuf is known and thus it can spare a strlen() call, and much more so than: strbuf_addf(&sb, "%s", sb2.buf); because it can spare the whole vsnprintf() formatting magic. Add new semantic patches to 'contrib/coccinelle/strbuf.cocci' to catch these undesired patterns and to suggest strbuf_addbuf() instead. Luckily, our codebase is already clean from any such undesired patterns (but one of the in-flight topics just tried to sneak in such a strbuf_addf() call). Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>