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.
• Finally, rebuild the repository summary table to correct the commit count.

Test Plan:

• Deleted a ref, ran pull + refs, saw oldref in database.
• Ran discover, saw it process the oldref, mark the unreachable commit, and update the summary table.
• Visited commit page, saw it properly marked.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9028

Differential Revision: https://secure.phabricator.com/D16133