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()