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:

    git@28c23cd

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Teng Long <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dyrone

contrib/coccinelle/strbuf.cocci

@@ -44,7 +44,7 @@ struct strbuf *SBP;
 
 @@
 expression E1, E2;
-format F =~ "s";
+format F =~ "^s$";
 @@
 - strbuf_addf(E1, "%@F@", E2);
 + strbuf_addstr(E1, E2);