Support hot-swapping of removable drives containing gcrypt repositories.

To support this, a core.gcrypt-id is stored by git-annex inside the git
config of a local gcrypt repository, when setting it up.

That is compared with the remote's cached gcrypt-id. When different, a
drive has been changed. git-annex then looks up the remote config for
the uuid mapped from the core.gcrypt-id, and tweaks the configuration
appropriately. When there is no known config for the uuid, it will refuse to
use the remote.

Remote/Web.hs

@@ -43,9 +43,9 @@ list = do
 	r <- liftIO $ Git.Construct.remoteNamed "web" Git.Construct.fromUnknown
 	return [r]
 
-gen :: Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> Annex Remote
+gen :: Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> Annex (Maybe Remote)
 gen r _ _ gc = 
-	return Remote {
+	return $ Just Remote {
 		uuid = webUUID,
 		cost = expensiveRemoteCost,
 		name = Git.repoDescribe r,