global: mark code units that generate warnings with -Wsign-compare

Mark code units that generate warnings with `-Wsign-compare`. This
allows for a structured approach to get rid of all such warnings over
time in a way that can be easily measured.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/add.c

@@ -3,6 +3,9 @@
  *
  * Copyright (C) 2006 Linus Torvalds
  */
+
+#define DISABLE_SIGN_COMPARE_WARNINGS
+
 #include "builtin.h"
 #include "advice.h"
 #include "config.h"