t/helper: mark unused callback void data parameters

Many callback interfaces have an extra void data parameter, but we don't
always need it (especially for dumping functions like the ones in test
helpers). Mark them as unused to avoid -Wunused-parameter warnings.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-dump-split-index.c

@@ -7,7 +7,7 @@
 #include "split-index.h"
 #include "ewah/ewok.h"
 
-static void show_bit(size_t pos, void *data)
+static void show_bit(size_t pos, void *data UNUSED)
 {
 	printf(" %d", (int)pos);
 }

t/helper/test-oid-array.c

@@ -4,7 +4,7 @@
 #include "setup.h"
 #include "strbuf.h"
 
-static int print_oid(const struct object_id *oid, void *data)
+static int print_oid(const struct object_id *oid, void *data UNUSED)
 {
 	puts(oid_to_hex(oid));
 	return 0;