Merge branch 'kk/commit-reach-optim' into jch

Merge branch 'kk/commit-reach-optim' into jch

The check for non-stale commits in the priority queue used by
`paint_down_to_common` and `ahead_behind` has been optimized by
replacing an O(N) scan with an O(1) counter, yielding performance
improvements in repositories with wide histories.

* kk/commit-reach-optim:
  commit-reach: replace queue_has_nonstale() scan with O(1) tracking
  commit-reach: deduplicate queue entries in paint_down_to_common
  object.h: fix stale entries in object flag allocation table