make manifest creation independent of the state of the local filecontent-metadata repository

(Also) register keys fetched from git to the key_registry. This eg. avoids the scenario that in a first run, a key is (still) new and would be added, but then in any subsequent run the key is already in the (local) git repository, thus it is no longer new, and is thus subsequently not added to the manifest...

should be merged together with Moore!1775 (merged)