Merge branch 'jt/commit-graph-per-object-store'

The singleton commit-graph in-core instance is made per in-core repository instance. * jt/commit-graph-per-object-store: commit-graph: add repo arg to graph readers commit-graph: store graph in struct object_store commit-graph: add free_commit_graph commit-graph: add missing forward declaration object-store: add missing include commit-graph: refactor preparing commit graph