pack-object: slightly more efficient

Avoid creating a delta index for objects with maximum depth since they
are not going to be used as delta base anyway.  This also reduce peak
memory usage slightly as the current object's delta index is not useful
until the next object in the loop is considered for deltification. This
saves a bit more than 1% on CPU usage.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

delta.h

@@ -18,6 +18,8 @@ create_delta_index(const void *buf, unsigned long bufsize);
 
 /*
  * free_delta_index: free the index created by create_delta_index()
+ *
+ * Given pointer must be what create_delta_index() returned, or NULL.
  */
 extern void free_delta_index(struct delta_index *index);