Merge branch 'ak/commit-graph-to-slab'

A few fields in "struct commit" that do not have to always be present have been moved to commit slabs. * ak/commit-graph-to-slab: commit-graph: minimize commit_graph_data_slab access commit: move members graph_pos, generation to a slab commit-graph: introduce commit_graph_data_slab object: drop parsed_object_pool->commit_count