object.h: Add OBJECT_ARRAY_INIT macro and make use of it.

Signed-off-by: Thiago Farina <tfransosi@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/fast-export.c b/builtin/fast-export.c
index 834ec8b..ab792d5 100644
--- a/builtin/fast-export.c
+++ b/builtin/fast-export.c
@@ -565,7 +565,7 @@
 int cmd_fast_export(int argc, const char **argv, const char *prefix)
 {
 	struct rev_info revs;
-	struct object_array commits = { 0, 0, NULL };
+	struct object_array commits = OBJECT_ARRAY_INIT;
 	struct string_list extra_refs = STRING_LIST_INIT_NODUP;
 	struct commit *commit;
 	char *export_filename = NULL, *import_filename = NULL;