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>

t/unit-tests/lib-reftable.c

@@ -1,3 +1,5 @@
+#define DISABLE_SIGN_COMPARE_WARNINGS
+
 #include "lib-reftable.h"
 #include "test-lib.h"
 #include "reftable/constants.h"

t/unit-tests/t-example-decorate.c

@@ -1,4 +1,5 @@
 #define USE_THE_REPOSITORY_VARIABLE
+#define DISABLE_SIGN_COMPARE_WARNINGS
 
 #include "test-lib.h"
 #include "object.h"

t/unit-tests/t-prio-queue.c

@@ -1,3 +1,5 @@
+#define DISABLE_SIGN_COMPARE_WARNINGS
+
 #include "test-lib.h"
 #include "prio-queue.h"
 

t/unit-tests/t-reftable-readwrite.c

@@ -6,6 +6,8 @@ license that can be found in the LICENSE file or at
 https://developers.google.com/open-source/licenses/bsd
 */
 
+#define DISABLE_SIGN_COMPARE_WARNINGS
+
 #include "test-lib.h"
 #include "lib-reftable.h"
 #include "reftable/basics.h"

t/unit-tests/t-reftable-stack.c

@@ -6,6 +6,8 @@ license that can be found in the LICENSE file or at
 https://developers.google.com/open-source/licenses/bsd
 */
 
+#define DISABLE_SIGN_COMPARE_WARNINGS
+
 #include "test-lib.h"
 #include "lib-reftable.h"
 #include "reftable/merged.h"

t/unit-tests/t-trailer.c

@@ -1,3 +1,5 @@
+#define DISABLE_SIGN_COMPARE_WARNINGS
+
 #include "test-lib.h"
 #include "trailer.h"
 

t/unit-tests/test-lib.c

@@ -1,3 +1,5 @@
+#define DISABLE_SIGN_COMPARE_WARNINGS
+
 #include "test-lib.h"
 
 enum result {