cocci: allow padding with `strbuf_addf()`

A convenient way to pad strings is to use something like
`strbuf_addf(&buf, "%20s", "Hello, world!")`.

However, the Coccinelle rule that forbids a format `"%s"` with a
constant string argument cast too wide a net, and also forbade such
padding.

The original rule was introduced by commit:

    28c23cd4c39 (strbuf.cocci: suggest strbuf_addbuf() to add one strbuf to an other, 2019-01-25)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Teng Long <dyroneteng@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
