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:

    28c23cd4c3 (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>

contrib/coccinelle/strbuf.cocci

@@ -15,7 +15,7 @@ constant fmt !~ "%";
 @@
 expression E;
 struct strbuf SB;
-format F =~ "s";
+format F =~ "^s$";
 @@
 - strbuf_addf(E, "%@F@", SB.buf);
 + strbuf_addbuf(E, &SB);
@@ -23,7 +23,7 @@ format F =~ "s";
 @@
 expression E;
 struct strbuf *SBP;
-format F =~ "s";
+format F =~ "^s$";
 @@
 - strbuf_addf(E, "%@F@", SBP->buf);
 + strbuf_addbuf(E, SBP);
@@ -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);