This handles the workflow where the branch is first pushed to the proxy,
and then files in the exported tree are later are copied to the proxied remote.
Turns out that the way the export log is structured, nothing needs
to be done to finalize the export once the last key is sent to it. Which
is great because that would have been a lot of complication. On
receiving the push, Command.Export runs and calls recordExportBeginning,
does as much as it can to update the export with the files currently
on it, and then calls recordExportUnderway. At that point, the
export.log records the export as "complete", but it's not really. And
that's fine. The same happens when using `git-annex export` when some
files are not available to send. Other repositories that have
access to the special remote can already retrieve files from it. As
the missing files get copied to the exported remote, all that needs
to be done is record each in the export db.
At this point, proxying to exporttree=yes annexobjects=yes special remotes
is fully working. Except for in the case where multiple files in the
tree use the same key, and the files are sent to the proxied remote
before pushing the tree.
It seems that even special remotes without annexobjects=yes will work if
used with the workflow where the git-annex branch is pushed before
copying files. But not with the `git-annex push` workflow.
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