Memory leaks in various codepaths have been plugged.

* ma/leakplugs:
  pack-bitmap[-write]: use `object_array_clear()`, don't leak
  object_array: add and use `object_array_pop()`
  object_array: use `object_array_clear()`, not `free()`
  leak_pending: use `object_array_clear()`, not `free()`
  commit: fix memory leak in `reduce_heads()`
  builtin/commit: fix memory leak in `prepare_index()`