Changing fork of extant dependency confuses wit update

[hcook]

If I have a grandchild dependency checked out, but then its parent (my child) dependency's manifest was changed to point to a fork of that grandchild repo, seems that wit update tries to find the target commit in the original (already present?) grandchild repo instead of fetching the forked grandchild repo and looking for it there, and so fails.

If I manually delete the grandchild dependency from the workspace, then wit update correctly refetches it and the target commit from the private fork.