*.c static functions: add missing __attribute__((format))

Add missing __attribute__((format)) function attributes to various
"static" functions that take printf arguments.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -213,6 +213,7 @@ static int used_atom_cnt, need_tagged, need_symref;
  * Expand string, append it to strbuf *sb, then return error code ret.
  * Allow to save few lines of code.
  */
+__attribute__((format (printf, 3, 4)))
 static int strbuf_addf_ret(struct strbuf *sb, int ret, const char *fmt, ...)
 {
 	va_list ap;