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/am.c

@@ -5,6 +5,8 @@
  */
 
 #define USE_THE_REPOSITORY_VARIABLE
+#define DISABLE_SIGN_COMPARE_WARNINGS
+
 #include "builtin.h"
 #include "abspath.h"
 #include "advice.h"