Merge branch 'tb/commit-graph-genv2-upgrade-fix' into maint

There was a bug in the codepath to upgrade generation information in commit-graph from v1 to v2 format, which has been corrected. source: <cover.1657667404.git.me@ttaylorr.com> * tb/commit-graph-genv2-upgrade-fix: commit-graph: fix corrupt upgrade from generation v1 to v2 commit-graph: introduce `repo_find_commit_pos_in_graph()` t5318: demonstrate commit-graph generation v2 corruption