cleanup: delete the reference-transaction hook

[arxanas]

I'll note that the long-term move is probably to stop using the reference-transaction hook, as it's problematic both fundamentally and incidentally:

• You have to walk the full event log to get the current reference locations.
• It's not actually possible to ensure atomic reference-transaction updates using the hook.
  • If we subscribe to the prepared hook, then there's no guarantee that the transaction will actually be committed.
  • If we subscribe to the committed hook, then it's possible that the hook call gets interrupted and we lose the update.
  • Occasionally my undo log shows stale branches that don't actually exist on disk anymore, which I think arises because of bad reference-transaction hook handling.
• Git currently doesn't group updates into reference transactions very well, so big fetches of $n$ branches induce $O(n)$ individual reference-transaction hook calls, which is incredibly slow and annoying.
  • For example: Speeding up performance #1269 (reply in thread) 😬
  • I think Git itself doesn't currently have the ability to do batch transactional updates, at least not without migrating to the reftable format fully?

I think the jj approach of snapshotting all the references every time is the only tractable long-term approach. At present, we actually walk all the references anyways to update our internal commit graph, so it's a cost we basically already incur.

Originally posted by @arxanas in #1322 (review)

---

To implement:

• Update WorkingCopySnapshot to include a list of all the references in the repository and their locations.
  • Would need to preserve symbolic vs direct reference information.
  • Branches have additional configuration in .git/config that needs to be copied/preserved.
  • The serialized information could be stored in a blob or tree. But it eventually needs to go into a commit object and included like the others:
    • in the commit message as metadata via one of the trailers
    • as a parent of the snapshot commit, so that Git GC will keep it live
  • We currently store head reference information separately; it may or may not be feasible to unify it with the other reference information.
• Update WorkingCopySnapshot to be able to restore all reference positions and all branch configuration.
• Replace the reference-transaction hook installation with a no-op, or delete it.
  • Should probably be careful not to break existing users whose hooks call into the subcommand; we might just want to change it to print a deprecation warning.
• Update the implementations of EventReplayer::get_cursor_main_branch_oid and EventReplayer::get_cursor_branch_oid_to_names to find the most recent snapshot and load reference positions from there.