Invent a way to retain reflogs for a while after the ref was deleted

[dscho]

It is a bit of a sad story that reflogs are deleted the moment the ref is deleted. So there is no easy "undo" there, even when the user has an "Ooops, I needed that branch and its reflog!" moment.

This will require a sane design around ref-less reflogs that can be garbage-collected after a configurable time.

[LemmingAvalanche]

Mentioned topic https://lore.kernel.org/git/[email protected]/T/#m746eaa03c795e401982be1509f6da73caaffdb3d

While I think this is sufficient motivation to have such a subcommand
nowadays already, I think the need for such a thing will grow in the
future. It was noted in multiple threads that we may eventually want to
lift the artificial limitations in the "reftable" backend where reflogs
are deleted together with their refs. This limitation is inherited from
the "files" backend, which may otherwise hit issues with directory/file
conflicts if it didn't delete reflogs.

Once that limitation is lifted for the "reftable" backend though, it
will become even more important to give users the tools to discover
reflogs that do not have a corresponding ref.