Finally gotten started on the borg special remote idea. A prerequisite of
that is remotes that can be imported from, but not exported to. So I
actually started by allowing setting importtree=yes without
exporttree=yes. A lot of code had assumptions about that not being allowed,
so it took a while to chase down everything. Finished most of that yesterday.

What I've done today is added a `thirdPartyPopulated` type of remote,
which `git-annex sync` can "pull" from by using the existing import
interface to list files on it, and determine which of them are annex object
files. I have not started on the actual borg remote at all, but this should
be all the groundwork for it done.

(I also finished up annex.stalldetection earlier this week.)

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This work was sponsored by Jake Vosloo [on Patreon](https://patreon.com/joeyh).