Support special remotes that are configured with importtree=yes but without exporttre...

Support special remotes that are configured with importtree=yes but without exporttree=yes

There was no particular reason not to support this, other than maybe a lack
of a use case. One use case would of course be a remote that you want to
avoid overwriting content on. A new use case is the idea of importing from
backups, eg borg, where exporting is not necessarily supported at all.

This commit was sponsored by Brock Spratlen on Patreon.