add a UUID to pair requests

Pair requests the the same UUID are part of the same pairing session,
which allows us to detect attempts to brute force the shared secret,
as that will result in pair requests with the same UUID that are
not verified with the right secret.

Assistant/Pairing.hs

@@ -7,6 +7,7 @@
 
 module Assistant.Pairing where
 
+import Common.Annex
 import Utility.Verifiable
 import Assistant.Ssh
 
@@ -49,6 +50,7 @@ data PairData = PairData
 	, remoteUserName :: UserName
 	, remoteDirectory :: FilePath
 	, remoteSshPubKey :: SshPubKey
+	, pairUUID :: UUID
 	}
 	deriving (Eq, Read, Show)