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

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

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