Merge branch 'tb/commit-graph-verify-fix'

The commit-graph verification code that detects mixture of zero and
non-zero generation numbers has been updated.

* tb/commit-graph-verify-fix:
  commit-graph: avoid repeated mixed generation number warnings
  t/t5318-commit-graph.sh: test generation zero transitions during fsck
  commit-graph: verify swapped zero/non-zero generation cases
  commit-graph: introduce `commit_graph_generation_from_graph()`