Merge branch 'rs/describe-with-lazy-queue-and-oidset'

Merge branch 'rs/describe-with-lazy-queue-and-oidset'

Instead of scanning for the remaining items to see if there are
still commits to be explored in the queue, use khash to remember
which items are still on the queue (an unacceptable alternative is
to reserve one object flag bits).

* rs/describe-with-lazy-queue-and-oidset:
  describe: use oidset in finish_depth_computation()