*.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>

merge-recursive.c

@@ -167,6 +167,7 @@ static void flush_output(struct merge_options *opt)
 	}
 }
 
+__attribute__((format (printf, 2, 3)))
 static int err(struct merge_options *opt, const char *err, ...)
 {
 	va_list params;