__attribute__: add a few missing format attributes

A public function mem_pool_strfmt() takes printf like parameters,
but is not given an attribute as such.  Also a few file-scope static
functions were missing their format attribute.

Add them.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

scalar.c

@@ -289,6 +289,7 @@ static int unregister_dir(void)
 }
 
 /* printf-style interface, expects `<key>=<value>` argument */
+__attribute__((format (printf, 1, 2)))
 static int set_config(const char *fmt, ...)
 {
 	struct strbuf buf = STRBUF_INIT;