sync, assistant, remotedaemon: Use ssh connection caching for git pushes and pulls.

For sync, saves 1 ssh connection per remote. For remotedaemon, the same
ssh connection that is already open to run git-annex-shell notifychanges
is reused to pull from the remote.

Only potential problem is that this also enables connection caching
when the assistant syncs with a ssh remote. Including the sync it does
when a network connection has just come up. In that case, cached ssh
connections are likely to be stale, and so using them would hang.
Until I'm sure such problems have been dealt with, this commit needs to
stay on the remotecontrol branch, and not be merged to master.

This commit was sponsored by Alexandre Dupas.

RemoteDaemon/Common.hs

@@ -20,7 +20,7 @@ import Annex.CatFile
 import Control.Concurrent
 
 -- Runs an Annex action. Long-running actions should be avoided,
--- since only one liftAnnex can be running at a time, amoung all
+-- since only one liftAnnex can be running at a time, across all
 -- transports.
 liftAnnex :: TransportHandle -> Annex a -> IO a
 liftAnnex (TransportHandle _ annexstate) a = do