__attribute__: mark some functions with LAST_ARG_MUST_BE_NULL

Some varargs functions that use NULL-terminated parameter list were
missing __attributes__ ((sentinel)) aka LAST_ARG_MUST_BE_NULL.

Add them.

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

scalar.c

@@ -70,6 +70,7 @@ static void setup_enlistment_directory(int argc, const char **argv,
 	strbuf_release(&path);
 }
 
+LAST_ARG_MUST_BE_NULL
 static int run_git(const char *arg, ...)
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;