Mesh migration does not mark migrated forks without objects as dirty

Issue Summary

When BBS migrates a fork to Mesh that does not have any objects or packs under repo-dir/objects, Mesh does not mark that repository as 'dirty' after migration.

The result is that the fork isn't vacuumed into shared-objects until the first write against the fork happens. If the fork is not used, but contains a ref to an object that is not referenced by any other repository in the hierarchy, its refs can become invalid after the shared-objects repository is garbage collected.

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available