Merge branch 'ps/do-not-trust-commit-graph-blindly-for-existence'

The codepath to traverse the commit-graph learned to notice that a commit is missing (e.g., corrupt repository lost an object), even though it knows something about the commit (like its parents) from what is in commit-graph. * ps/do-not-trust-commit-graph-blindly-for-existence: commit: detect commits that exist in commit-graph but not in the ODB commit-graph: introduce envvar to disable commit existence checks