To integrate with git-remote-gcrypt, a key thing is to have a way to map
from the gcrypt-id of an encrypted repository to a git-annex repository
uuid.

To do this, we'll make a v5 UUID, feeding in the gcrypt-id.
The namespace used is itself a v5 UUID, generated using the URL
namespace and the URL of this page at the time this scheme was
developed: "http://git-annex.branchable.com/design/gcrypt/"