odb: move logic to disable ref updates into repo

Our object database sources have a field `disable_ref_updates`. This field can obviously be set to disable reference updates, but it is somewhat curious that this logic is hosted by the object database. The reason for this is that it was primarily added to keep us from accidentally updating references while an ODB transaction is ongoing. Any objects part of the transaction have not yet been committed to disk, so new references that point to them might get corrupted in case we never end up committing the transaction. As such, whenever we create a new transaction we set up a new temporary ODB source and mark it as disabling reference updates. This has one (and only one?) upside: once we have committed the transaction, the temporary source will be dropped and thus we clean up the disabled reference updates automatically. But other than that, it's somewhat misdesigned: - We can have multiple ODB sources, but only the currently active source inhibits reference updates. - We're mixing concerns of the refdb with the ODB. Arguably, the decision of whether we can update references or not should be handled by the refdb. But that wouldn't be a great fit either, as there can be one refdb per worktree. So we'd again have the same problem that a "global" intent becomes localized to a specific instance. Instead, move the setting into the repository. While at it, convert it into a boolean. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>