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

diff --cc Makefile
Simple merge

diff --cc cache.h
Simple merge

diff --cc commit.c
Simple merge

diff --cc config.c
Simple merge

diff --cc ref-filter.c
Simple merge