Merge branch 'kk/commit-reach-find-all-fix' into next

The early-exit optimization in 'paint_down_to_common()' has been gated on the queue being generation-ordered, fixing a bug where 'git merge- base' (without '--all') could return incorrect results on repositories with v1 commit graphs and clock skew. * kk/commit-reach-find-all-fix: commit-reach: guard !FIND_ALL early exit with generation ordering check t6600: add test for merge-base early exit with clock skew