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.

Annex/Index.hs

@@ -9,6 +9,7 @@
 
 module Annex.Index (
 	withIndexFile,
+	addGitEnv,
 ) where
 
 import qualified Control.Exception as E
@@ -23,24 +24,30 @@ import Annex.Exception
 withIndexFile :: FilePath -> Annex a -> Annex a
 withIndexFile f a = do
 	g <- gitRepo
-#ifdef __ANDROID__
-	{- This should not be necessary on Android, but there is some
-	 - weird getEnvironment breakage. See
-	 - https://github.com/neurocyte/ghc-android/issues/7
-	 - Use getEnv to get some key environment variables that
-	 - git expects to have. -}
-	let keyenv = words "USER PATH GIT_EXEC_PATH HOSTNAME HOME"
-	let getEnvPair k = maybe Nothing (\v -> Just (k, v)) <$> getEnv k
-	e <- liftIO $ catMaybes <$> forM keyenv getEnvPair
-	let e' = ("GIT_INDEX_FILE", f):e
-#else
-	e <- liftIO getEnvironment
-	let e' = addEntry "GIT_INDEX_FILE" f e
-#endif
-	let g' = g { gitEnv = Just e' }
+	g' <- liftIO $ addGitEnv g "GIT_INDEX_FILE" f
 
 	r <- tryAnnex $ do
 		Annex.changeState $ \s -> s { Annex.repo = g' }
 		a
 	Annex.changeState $ \s -> s { Annex.repo = (Annex.repo s) { gitEnv = gitEnv g} }
 	either E.throw return r
+
+addGitEnv :: Repo -> String -> String -> IO Repo
+addGitEnv g var val = do
+	e <- maybe copyenv return (gitEnv g)
+	let e' = addEntry var val e
+	return $ g { gitEnv = Just e' }
+  where
+	copyenv = do
+#ifdef __ANDROID__
+		{- This should not be necessary on Android, but there is some
+		 - weird getEnvironment breakage. See
+		 - https://github.com/neurocyte/ghc-android/issues/7
+		 - Use getEnv to get some key environment variables that
+		 - git expects to have. -}
+		let keyenv = words "USER PATH GIT_EXEC_PATH HOSTNAME HOME"
+		let getEnvPair k = maybe Nothing (\v -> Just (k, v)) <$> getEnv k
+		liftIO $ catMaybes <$> forM keyenv getEnvPair
+#else
+		liftIO getEnvironment
+#endif