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

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