update export db after rename from annexobjects location

update export db after rename from annexobjects location

This allows git-annex post-receive, on the first push to
the remote to see that it is able to get a key from it in
order to upload it back.

Also avoided actively checking if the source remote contains a key.
The location log is good enough. If the location log is wrong,
the export of that file will fail with an informative message.