Merge branch 'jn/per-repo-object-store-fixes'

Step #0 of a planned & larger series to make the in-core object store per in-core repository object. * jn/per-repo-object-store-fixes: replace-objects: evaluate replacement refs without using the object store push, fetch: error out for submodule entries not pointing to commits pack: make packed_git_mru global a value instead of a pointer