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