Merge branch 'ds/commit-graph-genno-fix'

Merge branch 'ds/commit-graph-genno-fix'

Fix incremental update of commit-graph file around corrected commit
date data.

* ds/commit-graph-genno-fix:
  commit-graph: prepare commit graph
  commit-graph: be extra careful about mixed generations
  commit-graph: compute generations separately
  commit-graph: validate layers for generation data
  commit-graph: always parse before commit_graph_data_at()
  commit-graph: use repo_parse_commit