object: add object_array initializer helper function

The object_array API has an OBJECT_ARRAY_INIT macro, but lacks a
function to initialize an object_array at a given location in memory.

Introduce `object_array_init()` to implement such a function.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object.h

@@ -57,6 +57,8 @@ struct object_array {
 
 #define OBJECT_ARRAY_INIT { 0 }
 
+void object_array_init(struct object_array *array);
+
 /*
  * object flag allocation:
  * revision.h:               0---------10         15             23------27