Merge branch 'jk/commit-graph-slab-clear-fix' into maint-2.43

Clearing in-core repository (happens during e.g., "git fetch --recurse-submodules" with commit graph enabled) made in-core commit object in an inconsistent state by discarding the necessary data from commit-graph too early, which has been corrected. * jk/commit-graph-slab-clear-fix: commit-graph: retain commit slab when closing NULL commit_graph