The external special remote protocol supports export tree, but not yet
import tree. There is a draft protocol extension.

My main concern about this is, will external special remotes pick good
ContentIdentifiers and will they manage the race conditions documented in
[[import_tree]]? Mistakes in these things can result in data loss, and it's
rather subtle stuff. --[[Joey]]

> It may be better to implement [[importtree_only_remotes]] and make
> a simpler protocol extension that supports that, rather than supporting
> both export and import tree together. --[[Joey]]
