Copying DistTrees ends up with dest-repo "sharing" source-repo subrepos in odd ways

[ggainey]

Describe the bug The fix for #2851 has a last use-case that leaves repos "sharing" DistributionTrees. This is when you copy a DistributionTree from repository-A, into repository-B - repo-B ends up with a shared-DistributionTree-A. If repo-A syncs, and its upstream has changed the content of its subrepos (only), then both repo-A and repo-B end up with the same subrepo content even tho repo-B doesn't know anything has changed.

See discussion in subrepo-problem doc, and in the comments in #2851.