Merge branch 'kk/commit-graph-topo-levels-fix'

The 'topo_levels' slab was propagated only to the topmost layer of a
split commit-graph chain, causing topological levels for commits in
base layers to be recomputed during incremental writes.  This has been
corrected.

* kk/commit-graph-topo-levels-fix:
  commit-graph: propagate topo_levels slab to all chain layers
  commit-graph: add trace2 instrumentation for generation DFS

diff --cc commit-graph.c
Simple merge

diff --cc t/t5324-split-commit-graph.sh
Simple merge