The checkpresent hook can return either True or, False, or fail with a message
if it cannot successfully check the remote. Currently for glacier, when
--trust-glacier is not set, it always returns False. Crucially, in the case
when a file is in glacier, this is telling git-annex it's not there, so copy
re-uploads it. This is not desirable; it breaks using glacier-cli to retreive
that file later, and it wastes money/bandwidth.
What if it instead, when the glacier inventory is missing a
file, it returns False. And when the glacier inventory has a file, unless
--trust-glacier is set, it *fails*.
The result would be:
* `git annex copy --to glacier` would only send things not listed in inventory. If a file is listed in the inventory, `copy`
would complain that --trust-glacier` is not set, and not re-upload the file.
* `git annex drop` would only trust that glacier has a file when --trust-glacier is set. Behavior unchanged.
* `git annex move --to glacier`, when the file is not listed in inventory, would send the file, and delete it locally. Behavior unchanged.
* `git annex move --to glacier`, when the file is listed in inventory, would only trust that glacier has the file when --trust-glacier is set
* `git annex copy --from glacier` / `git annex get`, when the file is located in glacier, would trust the location log, and attempt to get the file from glacier.
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