commit: free the right buffer in release_commit_memory

The index field in the commit object is used to find the buffer
corresponding to that commit in the buffer_slab. Resetting it first
means free_commit_buffer is not going to free the right buffer.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -365,8 +365,8 @@ struct object_id *get_commit_tree_oid(const struct commit *commit)
 void release_commit_memory(struct parsed_object_pool *pool, struct commit *c)
 {
 	set_commit_tree(c, NULL);
-	c->index = 0;
 	free_commit_buffer(pool, c);
+	c->index = 0;
 	free_commit_list(c->parents);
 
 	c->object.parsed = 0;