R1:1c63ac6a3adf

When a ref is moved or deleted, put it on a list; later, check for reachability

Summary:
Ref T9028. This allows us to detect when commits are unreachable:

- When a ref (tag, branch, etc) is moved or deleted, store the old thing it pointed at in a list.
- After discovery, go through the list and check if all the stuff on it is still reachable.
- If something isn't, try to follow its ancestors back until we find something that is reachable.
- Then, mark everything we found as unreachable.