automatically launch git repository repair

Added a RemoteChecker thread, that waits for problems to be reported with
remotes, and checks if their git repository is in need of repair.

Currently, only failures to sync with the remote cause a problem to be
reported. This seems enough, but we'll see.

Plugging in a removable drive with a repository on it that is corrupted
does automatically repair the repository, as long as the corruption causes
git push or git pull to fail. Some types of corruption do not, eg
missing/corrupt objects for blobs that git push doesn't need to look at.

So, this is not really a replacement for scheduled git repository fscking.
But it does make the assistant more robust.

This commit is sponsored by Fernando Jimenez.

Assistant/Sync.hs

@@ -33,6 +33,7 @@ import Assistant.NamedThread
 import Assistant.Threads.Watcher (watchThread, WatcherControl(..))
 import Assistant.TransferSlots
 import Assistant.TransferQueue
+import Assistant.RemoteProblem
 import Logs.Transfer
 
 import Data.Time.Clock
@@ -59,11 +60,14 @@ import Control.Concurrent
 reconnectRemotes :: Bool -> [Remote] -> Assistant ()
 reconnectRemotes _ [] = noop
 reconnectRemotes notifypushes rs = void $ do
-	rs' <- filterM (checkavailable . Remote.repo) rs
+	rs' <- liftIO $ filterM (Remote.checkAvailable True) rs
 	unless (null rs') $ do
 		modifyDaemonStatus_ $ \s -> s
 			{ desynced = S.union (S.fromList $ map Remote.uuid xmppremotes) (desynced s) }
 		failedrs <- syncAction rs' (const go)
+		forM_ failedrs $ \r ->
+			whenM (liftIO $ Remote.checkAvailable False r) $
+				remoteHasProblem r
 		mapM_ signal $ filter (`notElem` failedrs) rs'
   where
 	gitremotes = filter (notspecialremote . Remote.repo) rs
@@ -90,10 +94,6 @@ reconnectRemotes notifypushes rs = void $ do
 	signal r = liftIO . mapM_ (flip tryPutMVar ())
 		=<< fromMaybe [] . M.lookup (Remote.uuid r) . connectRemoteNotifiers
 			<$> getDaemonStatus
-	checkavailable r
-		| Git.repoIsLocal r || Git.repoIsLocalUnknown r =
-			liftIO $ doesDirectoryExist $ Git.repoPath r
-		| otherwise = return True
 
 {- Updates the local sync branch, then pushes it to all remotes, in
  - parallel, along with the git-annex branch. This is the same