strong verification on retrieval from annexobjects location

strong verification on retrieval from annexobjects location

The file in the annexobjects location may have been renamed from a
previously exported file that got deleted in a subsequent export.
Or it may be renamed to annexobjects temporarily before being renamed to
another name (to handle eg pairwise renames).

But, an exported file is not guaranteed to contain the content of the
key that the local repository last exported there. Another tree could
have been exported from elsewhere in the meantime.

So, files in annexobjects do not necessarily have the content of their
key. And so have to be strongly verified when retrieving. The same as
is done when retrieving exported files.