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

diff --cc refs.c
Simple merge

diff --cc submodule.c
Simple merge