Merge branch 'jk/initialization-fix-to-add-submodule-odb' into maint

We peek objects from submodule's object store by linking it to the
list of alternate object databases, but the code to do so forgot to
correctly initialize the list.

* jk/initialization-fix-to-add-submodule-odb:
  add_submodule_odb: initialize alt_odb list earlier