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.

CmdLine/GitAnnex.hs

@@ -1,6 +1,6 @@
 {- git-annex main program
  -
- - Copyright 2010-2013 Joey Hess <joey@kitenet.net>
+ - Copyright 2010-2014 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -12,6 +12,8 @@ module CmdLine.GitAnnex where
 import qualified Git.CurrentRepo
 import CmdLine
 import Command
+import Utility.Env
+import Annex.Ssh
 
 import qualified Command.Add
 import qualified Command.Unannex
@@ -193,4 +195,5 @@ run args = do
 #ifdef WITH_EKG
 	_ <- forkServer "localhost" 4242
 #endif
-	dispatch True args cmds gitAnnexOptions [] header Git.CurrentRepo.get
+	maybe (dispatch True args cmds gitAnnexOptions [] header Git.CurrentRepo.get)
+		(runSshCaching args) =<< getEnv sshCachingEnv