Changes made, though I'm not quite sure about this part:

> Could you add a test in tests/store-database.scm for this bit?
>
>> +        (when (derivation-path? real-file-name)
>> +          (register-derivation-outputs))

Should that be a separate test or an extension of "register-path"?
Currently I'm doing the latter. And it looks ugly. The C++ code also
checks the validity of derivation outputs (whether the outputs should
actually have those filenames), which I'd like to eventually add to
register-items, and it would cause an error with the way the test
currently is. But then again, ffffff...fff-fake is probably already not
the right filename to be generated for that file anyway. Should we try
to do things "the proper way" there?

> Could you send updated patches?