plumb assicated files through P2P protocol for updating transfer logs

ReadContent can't update the log, since it reads lazily. This part of
the P2P monad will need to be rethought.

Associated files are heavily sanitized when received from a peer;
they could be an exploit vector.

This commit was sponsored by Jochen Bartl on Patreon.

Annex/Notification.hs

@@ -7,7 +7,7 @@
 
 {-# LANGUAGE CPP #-}
 
-module Annex.Notification (NotifyWitness, notifyTransfer, notifyDrop) where
+module Annex.Notification (NotifyWitness, noNotification, notifyTransfer, notifyDrop) where
 
 import Annex.Common
 import Types.Transfer
@@ -21,6 +21,10 @@ import qualified DBus.Client
 -- Witness that notification has happened.
 data NotifyWitness = NotifyWitness
 
+-- Only use when no notification should be done.
+noNotification :: NotifyWitness
+noNotification = NotifyWitness
+
 {- Wrap around an action that performs a transfer, which may run multiple
  - attempts. Displays notification when supported and when the user asked
  - for it. -}