An empty list of [ContentIdenfier] serialized to the same thing
as a single ContentIdentifier "". Avoid this ambiguity by requiring the
list be non-empty.
InodeCache serializes to a value with spaces, and seems likely other
things will too, and want to avoid unncessary base64 of content
identifiers when possible.
Does not yet have a way to update with new information from the
git-annex branch, which will be needed when multiple repos are importing
from the same remote.
