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