Merge branch 'jk/commit-graph-slab-clear-fix'

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

diff --cc commit-graph.c
Simple merge

diff --cc t/t5510-fetch.sh
Simple merge