Use ExportTree rather than ExportedLocation for retrieveKeyFile and
checkPresent. When another remote exported the content, ExportTree will
be populated, but ExportedLocation will not be.
It would be possible to implement storeKey to exports as well, but it
risks performing a lot of unncessary work when another repository
already stored the key on the export and the local repository doesn't
know about it.
The only way to avoid that work would be for storeKey to use checkPresentExport
before uploading. But, the other repository could have changed the
exported tree as well, so that can't be trusted, and if it were used in
storeKey, could result in bad information getting into the location log.
This commit was sponsored by Bruno BEAUFILS on Patreon.
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